UNIVERSITY  OF  CALIFORNIA 

AT   LOS  ANGELES 


TREATISE 


METEOROLOGY. 


WITH  A  COLLECTION  OF 


METEOROLOGICAL  TABLES, 


BY  ELIAS  LOOMIS,  LL.D,, 

PEOFE680B  OF  SATURAL  PHILOSOPHY  AND  ASTRONOMY  IN  YALE  COLLEGE,  AJTD  AUTHOR  OF  A 
"COURSE  OF  MATHEMATICS." 


NEW    YORK: 

HARPER    &    BROTHERS,    PUBLISHERS, 
327   TO   335   PEARL   STREET, 

FBAKKLIN     SQUAB  E. 

1894 


72) 


LOOMIS'S  SERIES  OF  TEXT-BOOKS. 


ELEMENTARY  ARITHMETIC.  166  pp.,  98  cent*. 
TREATISE  ON  ARITHMETIC.  352  pp.,  88  cent*. 
ELEMENTS  OF  ALGEBRA.  Revised  Edition.  281  pp.,  90  cents. 

Key  to  Element!  of  Algebra,  for  Use  of  Teacher*.     128  pp.,  90  cent*. 
TREATISE  ON  ALGEBRA.    Revised  Edition.    384  pp.,  $1  00. 

Key  to  Treatlte  on  Algebra,  tor  U»e  of  Teachers.    S19  pp.,  $1  00. 
ALGEBRAIC  PROBLEMS  AND  EXAMPLES.     S58  pp.,  90  cents. 
ELEMENTS  OF  GEOMETRY.     Revised  Edition.    388  pp.,  $1   00. 
ELEMENTS  OF  TRIGONOMETRY,   SURVEYING,  AND  NAVIGATION.    194  pp.,  $1  CO. 
TABLES  OF  LOGARITHMS.     150  pp.,  |1  00. 

The  Trigonometry  and   Tablet,  bound  in  one  volume.    360  pp.,  $1  50. 
ELEMENTS  OF  ANALYTICAL  GEOMETRY.    Revised  Edition.    261  pp.,  $1  00. 
DIFFERENTIAL  AND  INTEGRAL  CALCULUS.    Revised  Edition.    S09  pp.,  $1  00. 

The  Analytical  Geometry  and  Calculus,  bound  in  one  volume.    570  pp.,  $.1  "5. 
ELEMENTS  OF  NATURAL  PHILOSOPHY.    351  pp.,  $1  05. 
ELEMENTS  OF  ASTRONOMY.    854  pp.,  $1  00. 
PRACTICAL  ASTRONOMY.    499  pp.,  $1  60. 
TREATISE  ON  ASTRONOMY.    351  pp.,  $1  80. 
TREATISE  ON  METEOROLOGY.    308  pp.,  (1  50. 


Entered,  according  to  Act  of  Congress,  in  the  year  1868,  by 
HARPER    &    BROTHERS, 

In  the  Clerk's  Office  of  the  District  Court  of  the  United  States  for  the  Southern 
District  of  New  York. 


PREFACE. 


WITHIN  the  past  forty  years  a  vast  amount  of  meteorological 
observations  has  been  accumulated  from  almost  every  part  of  the 
world,  and  particularly  from  the  United  States.  Within  the  lim- 
its of  our  own  country  we  have  observations,  more  or  less  exten- 
sive, from  more  than  a  thousand  stations,  and  some  of  these  reg- 
isters are  very  accurate  and  complete.  So  great  an  amount  of 
labor  expended  upon  observations  ought  certainly  to  lead  to 
some  valuable  results.  Such  results  have  already  been  in  part 
attained,  but  they  are  generally  published  in  very  large  works, 
or  in  elaborate  memoirs  whose  object  is  limited  to  the  discus- 
sion of  special  questions.  Many  of  these  memoirs  are  only  to  be 
found  in  foreign  languages,  and  nearly  all  of  them  are  too  elab- 
orate to  circulate  freely  even  among  the  mass  of  tolerably  intelli- 
gent observers.  It  will  probably  be  conceded  that  there  has  not 
hitherto  appeared,  at  least  in  the  English  language,  any  general 
treatise  on  Meteorology  which  furnishes  a  comprehensive  view 
of  the  present  condition  of  every  branch  of  this  science  with  a 
minuteness  sufficient  to  satisfy  one  who  is  himself  engaged  in  the 
business  of  observing.  In  the  present  volume  an  attempt  has 
been  made  to  furnish  a  concise  exposition  of  the  principles  of 
Meteorology  in  a  form  adapted  to  use  as  a  text-book  for  instruc- 
tion, and  at  the  same  time  to  exhibit  the  most  important  results 
of  recent  researches.  That  this  attempt  has  been  but  partially 
successful  no  one  can  be  more  fully  aware  than  the  author ;  nev- 
ertheless it  is  hoped  that  this  volume  will  compare  favorably  with 
any  work  which  has  hitherto  appeared  having  the  same  objects 
in  view.  This  treatise  has  been  in  contemplation  for  many  years, 
during  which  I  have  been  collecting  materials  for  this  purpose. 


iv  PREFACE. 

It  would  have  been  quite  easy  to  have  expanded  the  book  to 
double  its  present  size,  and  in  such  a  form  it  might  have  been 
more  satisfactory  to  those  who  are  themselves  engaged  in  original 
researches;  but  I  have  aimed  to  prepare  a  work  which  should 
not  only  be  useful  to  observers,  but  should  also  be  adapted  to 
purposes  of  instruction  in  our  colleges  and  scientific  schools.  It 
is  hoped  that  this  volume  may  serve  to  stimulate  observers,  by 
showing  them  the  important  results  already  deduced  from  their 
labors,  and  also  by  calling  their  attention  to  the  unsettled  prob- 
lems which  require  for  their  solution  either  more  accurate  or 
more  numerous  observations. 

I  have  again  to  acknowledge  my  obligations  to  Professor  II. 
A.Newton,  who  has  read  all  the  proofs  of  this  work,  and  to  whom 
I  am  indebted  for  numerous  suggestions,  particularly  in  the  last 
chapter,  which  relates  to  a  subject  to  which  he  has  devoted  spe- 
cial attention. 


CONTENTS. 


CHAPTER  I. 

CONSTITUTION    AND    WEIGHT   OF   THE    ATMOSPHERE. 

P»g» 

Composition  of  the  Air — Dalton's  Theory  of  the  Atmosphere 9 

Construction  of  the  Barometer — Corrections  for  Temperature,  etc 12 

Self-registering  Barometers — Mean  Height  of  the  Barometer 15 

Inequality  of  the  Monthly  Means — Hourly  Variations 19 

Extreme  Fluctuations  of  the  Barometer — Heights  measured  by  the  Barometer  ...  21 

CHAPTER  II. 

TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH. 

Construction  of  the  Thermometer — Graduation,  etc 23 

Self-registering  Thermometers — Proper  Exposure  of  a  Thermometer 25 

Hourly  Variations  of  Temperature — Mean  Temperature  of  a  Day  determined 29 

Mean  Temperature  of  the  Months — Mean  Temperature  of  a  Place 31 

Distribution  of  Heat  over  the  Earth's  Surface — Isothermal  Lines 34 

Two  Sides  of  the  Atlantic  compared — Hottest  and  Coldest  Months 37 

Highest  and  Lowest  observed  Temperatures 39 

Temperature  of  the  Air  at  different  Heights — Limit  of  perpetual  Snow 40 

Temperature  of  Earth  at  different  Depths — Time  of  Maximum  and  Minimum....  44 

Information  furnished  by  Volcanoes,  Hot  Springs,  etc —A.  47 

Natural  Ice-houses — Temperature  of  the  Sea 49 

Temperature  of  Banks — Polar  Ice — Anchor  Ice 51 

CHAPTER  III. 

THE    MOISTURE    OF    THE    AIR. 

How  Vapor  is  sustained  in  the  Air — Amount  of  Evaporation  measured 54 

Hygrometers — Saussure's — Bache's — Daniell's,  etc 56 

Weight  of  Vapor  determined — Diurnal  Variation  in  Amount  of  Vapor 60 

Annual  Variation  of  Pressure  of  the  Gaseous  Atmosphere 63 

CHAPTER  IV. 

THE    MOTIONS    OF   THE    ATMOSPHERE. 

How  to  determine  the  Direction  of  the  Wind.... 65 

Self-registering  Anemometers — Whewell's — Robinson's — Osier's,  etc 66 

Average  Velocity  of  the  Wind — Mean  Direction  of  the  Wind „ 70 

The  Trade  Winds — Winds  in  the  Middle  Latitudes — Polar  Winds 74 

Motion  of  the  Upper  Half  of  the  Atmosphere 76 

Causes  of  the  Winds — Mode  of  Propagation  of  Winds 79 

Cause  of  the  high  Barometer  near  the  Parallel  of  32° 84 

The  Monsoons — Land  and  Sea  Breezes — Hot  Winds  of  Deserts. ..,  ....  85 


Vi  CONTENTS. 

CHAPTER  V. 

PRECIPITATION   OF   THE   VAPOR   OF   THE   AIR. 

SECTION  L—  DEW.  p 

Effect  of  Radiation  of  Heat— Origin  of  Dew 89 

Circumstances  favorable  to  Dew — Where  there  is  no  Dew 91 

SECTION  II.—  HOAR-FROST. 

Formation  of  Hoar-frost— Crystalline  Structure  of  Hoar-frost 93 

SECTION  III.— Foo. 

Fogs  over  Rivers  in  Summer — Fogs  in  Spring  and  Winter 95 

The  Vesicular  Theory — Diameter  of  Particles  of  Fog — Indian  Summer 98 

SECTION  IV.— CLOUDS. 

Classification  of  Clouds — Cirrus — Cumulus — Stratus,  etc 101 

Height  of  Clouds— Formation  of  Clouds 103 

Peculiar  Arrangement  of  Clouds — Shadows  of  Clouds 106 

SECTION  V.— RAIN. 

To  measure  the  Amount  of  Rain — Proper  Exposure  of  the  Gauge 108 

Hutton's  Theory  of  Rain — Distribution  of  Rain  over  the  Earth's  Surface Ill 

Influence  of  Elevation  above  the  Sea — Influence  of  Winds 114 

Annual  Fall  of  Rain  at  different  Places — Greatest  Fall  of  Rain 117 

Deserts — Rain  without  Clouds 119 

SECTION  VI.— SNOW. 

How  Flakes  of  Snow  are  formed — Form  of  Snow-flakes 122 

Red  Snow  in  the  Polar  Regions — Glaciers 126 

SECTION  VH.— HAIL. 

Size  of  Hailstones — Form  of  Hailstones 129 

Track  of  Hail-storms — Origin  of  the  Cold  which  causes  Hail 1 32 

Process  of  the  Formation  of  Hail — Hail-rods 134 

• 

CHAPTER  VI. 

STORMS,   TORNADOES,   AND   WATER-SPOUTS. 

SECTION  I THEORY  AND  LAWS  OF  STORMS. 

Cause  of  Storms — Why  the  Barometer  falls  under  a  Cloud 136 

Amount  of  the  Barometric  Depression — Gradual  Rise  and  Decline  of  Storms  ...  139 

Distinction  between  the  Direction  of  the  Wind  and  that  of  the  Storm's  Progress  143 

Course  of  Storms  modified  by  Local  Causes 145 

SECTION  II.— CYCLONES. 

Paths  of  Cyclones— Gyratory  Movement  of  the  Air  in  Cyclones 148 

Cause  of  the  Parabolic  Course  of  Storms 151 

SECTION  III.— TORNADOES. 

Tropical  Tornadoes — Effects  of  Tornadoes 152 

SECTION  IV.— PILLARS  op  SAND  AND  WATER-SPOUTS. 
Whirlwinds  caused  by  Fires — Water-spouts 154 

SECTION  V — PREDICTIONS  OF  TOE  WEATIIEB. 

Predictions  founded  on  the  Constancy  of  Climate 157 

Predictions  founded  on  the  established  Laws  of  Storms.. .,  ..  158 


CONTENTS.  Vii 

CHAPTER  VII. 

ELECTRICAL   PHENOMENA. 

SECTION  L— ATMOSPHEBIC  ELECTRICITY,  p,^ 

Electrometers — Diurnal  Variation  of  Electricity 160 

Origin  of  Atmospheric  Electricity — Electricity  in  dry  Houses 163 

SECTION  II.—  TncNDm-STORMS. 

Lightning — Different  Forms  of  Lightning 166 

Duration  of  Lightning — Cause  of  Thunder 168 

Rolling  of  Thunder— Height  of  Thunder  Clouds 170 

SECTION  III.— AUBOBA  POLABIS. 

Varieties  of  Aurora — Arches — Beams — Corona 174 

Geographical  Extent  of  Auroras — Auroral  Arches 177 

Structure  of  Auroral  Arches — the  Corona 181 

Height  of  the  Aurora — Noise  of  the  Aurora 184 

Geographical  Distribution  of  Auroras — Auroras  in  the  Southern  Hemisphere....  186 

Periodicity  of  Auroras — Diurnal — Annual  and  Secular 188 

Disturbance  of  the  Magnetic  Needle 190 

Theory  of  the  Polar  Light — The  Auroral  Light  is  Electric  Light 191 

What  are  Auroral  Beams? — Circulation  of  Electricity  about  the  Earth 194 

Cause  of  the  Magnetic  Disturbances — Cause  of  the  Periodicity 197 

Geographical  Distribution  explained — System  of  Electrical  Circulation 199 

CHAPTER  VHI. 

OPTICAL   METEOROLOGY. 
SECTION  L— MIBAOK. 

Mirage  upon  a  Desert — Mirage  at  Sea 202 

Lateral  Mirage — Displacements 205 

SECTION  II.—  ABSORPTION  OF  LIGHT  BY  TUB  ATMOSPHERE. 

Redness  of  the  Evening  Sky — Blue  Color  of  the  Sky 206 

Cyanometer — Twilight — Colors  of  the  Morning  Twilight 207 

SECTION  HI.— THE  RAINBOW. 

Dimensions  of  the  Rainbow  computed — Conditions  of  Visibility 210 

Supernumerary  Bows — Their  Theory  explained 211 

Size  of  the  Drops  of  Rain — Fog-bow  explained 213 

SECTION  IV.— CORONJL 

Order  of  Colors  in  Corona? — Cause  of  Coronas 214 

SECTION  V HALOS  AND  PARHELIA. 

Halo  of  22°  Radius— Theory  of  this  Halo 216 

Halo  of  46°  Radius— Halo  of  90°  Radius 218 

Parhelic  Circle— Parhelia 220 

Contact  Arches — Their  variable  Form 222 

Intersecting  Arcs  opposite  to  the  Sun — Vertical  Columns 224 

CHAPTER  LX. 

SHOOTING-STARS,   DETONATING   METEORS,  AND  AEROLITES. 
SECTION  I — SnooTiNO-STABS. 

Number  seen  at  different  Hours — Number  seen  in  the  different  Months 225 

Altitude  of  Shooting-stars — Length  of  Path  and  Velocity 226 

Cause  of  the  Light  of  Shooting-stars — Meteoric  Orbits 228 


Viii  CONTENTS. 

P«» 

Periodic  Meteors  of  November — Meteoric  Showers 230 

Period  of  the  November  Meteors — Elements  of  the  November  Meteors 233 

Periodic  Meteors  of  August — Elements  of  the  Orbit  of  the  August  Meteors 235 

SECTION  II.— DETONATIKG  METEOES. 

Examples  of  Detonating  Meteors — Number,  Velocity,  etc 238 

Periodicity  of  Detonating  Meteors 240 

SECTION  III.— AEBOLITES. 

Examples  of  Aerolites — Number  of  Aerolites 241 

Composition  of  Aerolites — Peculiarities  of  Aerolites 244 

Widmannstaten  Figures — Periodicity  of  Aerolites , 245 

Origin  of  Aerolites — Conclusions 247 

TABLES. 

I.  To  convert  Millimetres  into  English  Inches 251 

II.  To  convert  Metres  into  English  Feet 252 

III.  To  convert  Kilometres  into  English  Miles 253 

IV.  To  convert  French  Feet  into  English  Feet 254 

V.  To  compare  the  Centesimal  Thermometer  with  Fahrenheit's 255 

VI.  To  compare  Reaumur's  Thermometer  with  Fahrenheit's 256 

VII.  Column  of  Air  corresponding  to  one  tenth  Inch  in  the  Barometer 257 

VIII.  For  reducing  Barometric  Observations  to  the  Freezing  Point 258 

IX.  To  determine  Altitudes  with  the  Barometer 260 

X.  Mean  Height  of  the  Barometer  in  the  different  Months 262 

XL  Mean  Height  of  the  Barometer  for  all  Hours  of  the  Day 263 

XII.  Depression  of  Mercury  in  Glass  Tubes 263 

XIII.  To  compare  the  Weight  of  a  Cubic  Foot  of  Dry  Air  and  of  Saturated  Air  264 

XIV.  Height  of  Barometer  corresponding  to  Temperature  of  Boiling  Water  265 
XV.  Diurnal  Variation  of  Temperature  at  New  Haven,  Conn 266 

XVI.  Diurnal  Variation  of  Temperature  at  Greenwich,  Eng 267 

XVII.  Mean  Temperature  for  each  Month,  Season,  and  the  Year 268 

XVIII.  Places  whose  Mean  Temperature  is  above  80°  Fahrenheit 270 

XIX.  Places  whose  Mean  Temperature  is  below  18°  Fahrenheit 270 

XX.  Places  having  a  Small  Monthly  Range  of  Temperature 271 

XXI.  Places  having  a  Great  Monthly  Range  of  Temperature 271 

XXII.  Places  having  a  Small  Absolute  Range  of  Temperature 272 

XXIII.  Places  having  a  Large  Absolute  Range  of  Temperature 272 

XXIV.  Height  of  the  Snow  Line  above  the  Sea 273 

XXV.  Factors  for  Dry-bulb  and  Wet-bulb  Thermometers 273 

XXVI.  Relative  Humidity  of  the  Air 274 

XXVII.  Elastic  Force  of  Aqueous  Vapor 276 

XXVIII.  For  comparing  the  Pressure  and  Velocity  of  the  Wind 277 

XXIX.  Average  Amount  of  Rain  for  each  Month,  Season,  and  the  Year 278 

XXX.  Places  having  a  Small  Annual  Fall  of  Rain 280 

XXXI.  Places  having  a  Great  Annual  Fall  of  Rain 280 

XXXII.  Comparative  Radiating  Power  of  different  Substances  at  Night 281 

XXXIII.  Fall  of  the  Barometer  in  Hurricanes 281 

XXXIV.  Auroras,  Solar  Spots,  and  Variation  of  the  Magnetic  Needle 282 

XXXV.  Catalogue  of  the  largest  Iron  Meteors 283 

XXXVI.  Aerolites  fallen  in  the  United  States 284 

EXPLANATION  OF  THE  TABLES 285 

INDEX 301 


METEOROLOGY. 


CHAPTER  I. 

CONSTITUTION  AND  WEIGHT  OF  THE  ATMOSPHERE. 

1.  THE  term  meteor  was  formerly  employed  to  denote  those 
natural  phenomena  which  occur  within  the  limits  of  our  atmos- 
phere, as  the  wind,  rain,  thunder,  the  rainbow,  etc. ;  and  Meteor- 
ology might,  therefore,  be  defined  as  that  branch  of  Natural  Phi- 
losophy which  treats  of  Meteors. 

This  branch  of  science  treats  of  the  constitution  and  wtJght 
of  the  air;  of  its  temperature  and  moisture;  of  the  movements 
of  the  atmosphere ;  of  the  precipitation  of  vapor  in  the  form  of 
dew,  hoar-frost,  fog,  cloud,  rain,  snow,  and  hail ;  of  the  laws  of 
Storms,  including  tornadoes  and  water-spouts;  with  various  elec- 
trical phenomena,  including  atmospheric  electricity,  thunder- 
storms, and  the  Polar  Aurora ;  as  also  various  optical  phenome- 
na, including  the  rainbow,  twilight,  mirage,  corona,  and  halos ; 
to  which  are  generally  added  aerolites  and  shooting  stars. 

2.  Composition  of  the  Air. — Atmospheric  air  is  not  a  simple  sub- 
stance, as  was  once  believed,  but  consists  of  nitrogen  and  oxy- 
gen, together  with  more  or  less  vapor  of  water,  and  almost  always 
a  little  carbonic  acid.     The  nitrogen  and  oxygen  are  combined 
in  the  ratio  of  79.1  to  20.9  by  volume.     These  proportions  are 
generally  the  same  in  all  parts  of  the  globe,  and  at  all  accessible 
elevations  above  the  earth's  surface.     During  a  balloon  ascent, 
air  has  been  collected  from  an  elevation  of  21,774  feet,  and  its 
constitution  was  found  to  be  sensibly  the  same  as  that  of  air  at 
the  earth's  surface. 

Atmospheric  air  contains  a  little  carbonic  acid  (from  0.0004  to 
0.0006  in  the  open  country),  and  a  variable  amount  of  vapor  of 
water.  The  amount  of  moisture  in  the  atmosphere  sometimes 


10  METEOROLOGY. 

forms  four  per  cent,  of  its  entire  weight,  and  sometimes  is  less 
than  a  tenth  of  one  per  cent. 

3.  Distinction  between  Vapors  and  Gases. — Aeriform  bodies  are 
naturally  divided  into  two  classes.     Some  are  easily  reduced  to 
the  liquid  state,  and  are  called  vapors,  as  the  vapor  of  water. 
Others  always  remain  in  the  aeriform  state,  or  can  only  be  re- 
duced to  the  liquid  state  with  the  greatest  difficulty.     These  are 
called  gases,  such  as  oxygen,  nitrogen,  hydrogen,  etc. 

4.  Law  of  Mixture  of  Gases. — When  vapors  and  gases  are  super 
posed  upon  each  other,  they  obey  a  law  different  from  liquids. 
If  we  pour  into  the  same  vessel  several  liquids  which  exert  no 
chemical  action  upon  each  other,  they  will  arrange  themselves  in 
the  order  of  their  specific  gravities;  the  heaviest  will  subside  to 
the  bottom,  and  the  lightest  will  float  upon  the  surface.     But  if 
we  introduce  into  the  same  vessel  different  gases,  they  will  not 
arrange  themselves  in  separate  strata  in  the  order  of  their  specific 
gravities,  but  will  mutually  penetrate  each  other,  and  after  a  short 
time  the  proportion  of  the  several  gases  will  be  the  same  in  every 
part  of  the  vessel.     This  movement  of  gases  toward  each  other 
has  received  the  name  of  diffusion. 

5.  Daltoris  Theory  of  the  Atmosphere. — According  to  the  theory 
of  Dalton,  the  gases  which  compose  the  atmosphere  are  not  in  a 
state  of  chemical  combination,  but  the  particles  of  either  gas  have 
neither  attraction  nor  repulsion  for  those  of  another,  and  each  of 
them  is  disposed  precisely  as  if  the  others  were  not  present. '    He 
therefore  considered  that  the  earth  is  surrounded  in  effect  by  four 
atmospheres,  which  interpenetrate  each  other,  but  without  inter- 
ference. 

The  hypothesis  that  there  is  no  repulsion  between  the  particles 
of  the  different  gases  which  compose  the  atmosphere  has  not  been 
generally  received.  The  diffusion  of  gases  may  be  explained  by 
supposing  that  the  molecules  of  gases  are  situated  at  great  dis- 
tances from  each  other;  and  each  gas,  therefore,  presents  vast 
pores  through  which  the  particles  of  the  other  gas  may  penetrate. 

6.  Gases  in  the  upper  Regions  of  the  Atmosphere. — In  the  upper 
and  inaccessible  regions  of  the  atmosphere  there  are  no  othei 


CONSTITUTION   AND   WEIGHT    OF   THE   ATMOSPHERE.          11 

gases  than  those  found  at  the  surface  of  the  earth,  for  such  gases 
would  in  time  penetrate  to  the  earth's  surface  by  the  force  of 
diffusion.  The  hypothesis,  therefore,  which  explains  certain  fiery 
meteors  by  the  assumption  of  an  inflammable  gas  in  the  upper 
regions  of  the  atmosphere,  is  inadmissible. 

7.  Proportions  of  the  Gases  at  Great  Elevations. — A  stratum  of 
air  near  the  earth  sustains  the  weight  of  the  entire  superincum- 
bent atmosphere,  and  its  density  is  thereby  increased.     This  dens- 
ity diminishes  as  we  rise  above  the  surface  of  the  earth ;  and 
since  each  gas  is  distributed  as  if  no  other  gas  was  present,  this 
diminution  (which  depends  upon  the  weight  of  the  gas)  ought 
not  to  be  the  same  for  each  of  the  constituents  of  the  atmosphere. 
At  great  elevations,  the  proportion  of  these  gases  should  there- 
fore be  different  from  what  it  is  at  the  earth's  surface.     It  has 
been  computed  that,  at  the  height  of  four  miles,  the  proportion 
of  nitrogen  to  oxygen  should  be  one  per  cent,  greater  than  at  the 
earth's  surface.     Observation  has,  however,  shown  that  there  is 
no  such  difference,  a  result  which  is  attributed  to  the  constant 
agitation  of  the  atmosphere,  by  which  the  different  strata  are 
thoroughly  mingled  together. 

8.  Limit  of  the  Atmosphere  determined  by  Centrifugal  Force. — Since 
the  earth's  attraction,  which  retains  the  air  near  to  its  surface,  va- 
ries inversely  as  the  square  of  the  distance  from  the  centre,  while 
the  centrifugal  force  arising  from  the  earth's  rotation  increases 
with  the  distance,  there  must  be  a  certain  height  at  which  these 
two  forces  are  equal,  and  beyond  this  distance  the  air  will  be 
dissipated  by  centrifugal  force.     This  point  is  about  25,000  miles 
from  the  earth's  centre. 

9.  Estimate  of  the  Actual  Height  of  the  Atmosphere.  —  Other  con- 
siderations indicate  a  much  lower  limit  to  the  atmosphere.     The 
atmosphere  must  terminate  at  that  height  where  the  attraction 
of  the  earth  is  just  equal  to  the  repulsion  between  the  particles  of 
air,  and  this  repulsion  is  diminished  by  the  low  temperature  of 
the  upper  regions.     At  the  height  of  50  miles  the  atmosphere  is 
well-nigh  inappreciable  in  its  effect  upon  twilight.     The  phenom- 
ena of  lunar  eclipses  indicate  that  the  earth's  atmosphere  is  ap- 
preciable at  the  height  of  66  miles.     The  phenomena  of  shooting 


12 


METEOROLOGY. 


otars  and  the  auroral  light  indicate  that  an  appreciable  atmos- 
phere exists  at  the  height  of  200  or  300  miles,  and  probably 
more  than  500  miles  from  the  earth's  surface. 

10.  Construction  of  the  Barometer. — The  weight  of  the  atmosphere 

is  measured  by  a  barometer.  If  we  take  a  glass 
tube,  A  B,  about  three  feet  in  length,  hermetically 
sealed  at  one  end  and  open  at  the  other,  fill 
it  with  quicksilver,  and  then,  closing  the  open 
end  of  the  tube  with  the  finger,  invert  the  tube, 
and  immerse  the  lower  end  in  a  cup  filled  with 
mercury,  on  removing  the  finger  the  liquid  will 
fall  only  a  moderate  distance,  and  will  be  main- 
tained at  an  elevation  of  about  thirty  inches  above 
the  level  of  the  liquid  in  the  cup.  The  column 
of  mercury  in  the  tube  C  D  is  supported  by  the 
pressure  of  the  air  acting  on  the  surface  of  the 
mercury  in  the  cup ;  and  we  conclude  that  the 
weight  of  a  column  of  mercury  having  a  height 
of  thirty  inches  is  equal  to  that  of  a  column  of 
air  of  the  same  base,  extending  to  the  top  of  the  atmosphere. 
Such  an  instrument  is  called  a  Barometer.  The  barometer  meas- 
ures, therefore,  the  pressure  of  the  air,  and,  in  order  to  ascertain 
its  amount,  we  have  only  to  attach  to  the  glass  tube  a  graduated 
scale. 

In  order  to  allow  entire  freedom  of  motion  to  the  column  of 
mercury,  the  diameter  of  the  tube  should  not  be  too  small.  For 
a  stationary  barometer,  a  tube  having  an  internal  diameter  of  half 
an  inch  is  not  too  great 

11.  How  Air  and  Moisture  are  Excluded. — Special  care  should  be 
taken  to  exclude  from  the  tube  both  air  and  moisture,  the  pres- 
ence of  which  would  produce  pressure  upon  the  upper  extremity 
of  the  column  of  mercury,  and  depress  it  below  its  proper  height. 
It  is  found  very  difficult  to  attain  this  object  perfectly.     The  tube 
should  be  entirely  clean,  and  the  mercury  should  be  filtered,  and 
both  should  be  heated,  in  order  to  expel  moisture,    A  small  quan- 
tity of  mercury  is  then  introduced  into  the  tube,  special  care  be- 
ing taken  to  prevent  the  admission  of  air-bubbles.     The  tube  is 
then  held  over  a  charcoal  fire  and  heated  until  the  mercury  boils, 


CONSTITUTION  AND   WEIGHT  OF  THE  ATMOSPHERE. 


13 


the  tube  being  held  in  an  inclined  position,  so  that  any  particles 
which  may  adhere  to  the  sides  of  the  tube  may  easily  escape. 
More  mercury  is  now  added,  and  the  operation  of  boiling  repeat- 
ed as  before,  and  thus  the  tube  is  gradually  filled. 

If  a  barometer-tube  has  been  well  freed  from  air  and  moisture, 
when  the  tube  is  suddenly  inclined  the  mercury  will  strike  the 
top  of  the  tube  with  a  sharp  metallic  sound. 

12.  How  the  Height  of  the  Column  is  Measured. — The  height  of 
the  mercury  in  the  barometer  varies  from  day  to  day,  and  the 
graduation  of  the  scale  by  which  its  height  is  measured  should 
have  a  sufficient  range  to  comprehend  the  extreme  variations  in 
the  height  of  the  column.  With  a  stationary  barometer,  these 
variations  are  generally  comprehended  between  27  and  31  inches. 
This  portion  of  the  scale  is  divided  into  tenths  of  an  inch,  and 
these  spaces  are  still  farther  subdivided  by  means  of  a,  vernier. 

The  graduated  scale  may  be  either  faced  or  movable.  If  the  scale 
be  fixed,  a  correction  will  be  required  for  the  oscillations  of  the 
Fig. -2.  mercury  in  the  tube.  Suppose,  when  the  air  is  at  its 
mean  pressure,  the  lower  extremity  of  the  graduated 
scale  just  touches  the  surface  of  the  mercury  in  the 
cistern.  When  the  pressure  diminishes,  the  mercury 
which  descends  from  the  tube  fills  the  cistern  to  a 
greater  height,  and  its  level  rises  above  the  lower  ex- 
tremity of  the  scale.  When  the  pressure  increases, 
mercury  from  the  cistern  ascends  into  the  tube,  and 
its  level  is  left  below  the  extremity  of  the  scale.  Thus 
the  lower  extremity  of  the  graduated  scale  alternately 
sinks  below  the  level  of  the  cistern,  and  rises  above 
it,  in  neither  of  which  cases  is  the  true  pressure  of 
the  atmosphere  directly  indicated.  As,  however,  when 
we  know  the  relative  diameters  of  the  tube  and  cis- 
tern, the  variations  of  the  level  of  the  cistern  may 
be  easily  computed,  such  a  barometer  may  give  ac- 
curate results ;  yet  the  inconvenience  is  entirely  rem- 
edied by  making  the  scale  movable.  In  this  case  the 
lower  extremity  of  the  scale  is  made  to  terminate  in 
an  ivory  point,  which,  by  the  motion  of  a  screw,  D, 
may  at  each  observation  be  brought  to  exact  coinci- 
dence with  A,  the  surface  of  the  mercury  in  the  cistern 


14-  METEOROLOGY. 

In  some  barometers  the  scale  is  fixed,  but  the  level  of  the 
mercury  in  the  cistern  may  be  adjusted  to  the  extremity  of  the 
scale  by  means  of  a  screw,  B. 

In  order  that  observations  made  with  different  barometers  may 
be  comparable,  corrections  are  required  both  for  temperature  and 
for  capillary  action. 

13.  Correction  for  Temperature. — Heat  expands  the  column  of 
mercury ;  that  is,  diminishes  its  specific  gravity,  and  thus  a  great- 
er height  is  required  to  produce  a  given  pressure.     Now,  since 
the  barometer  is  daily  subjected  to  changes  of  temperature,  va- 
riations in  the  height  of  the  column  do  not  necessarily  indicate 
variations  of  pressure.     Before  we  can  decide  whether  there  has 
been  a  change  of  pressure,  we  must  compute  the  effect  due  to  the 
change  of  temperature.     For  this  purpose,  we  must  know  the  tem- 
perature of  the  mercury  at  each  observation;  and,  accordingly, 
a  thermometer  always  accompanies  a  barometer,  and  is  techni- 
cally called  the  attached  thermometer.     At  every  observation  of 
the  barometer  the  attached  thermometer  should  also  be  observed. 
For  the  purpose  of  comparison,  all  barometric  observations  should 
be  reduced  to  a  standard  temperature,  and  the  temperature  gener= 
ally  agreed  upon  is  that  of  melting  ice.     The  expansion  of  mer- 
cury from  the  temperature  of  melting  ice  to  that  of  boiling  water 
is  -5*5  of  its  volume,  which  is  about  -nr.ViRrth  part  for  one  degree 
of  Fahrenheit's  thermometer.      In  order,  therefore,  to  reduce  the 
observed  height  of  the  barometer  to  the  height  which  would  have 
been  indicated  if  its  temperature  had  been  32°,  we  must  subtract 
the  ten  thousandth  part  of  the  observed  altitude  for  each  degree 
above  the  freezing  point.     If  the  temperature  be  below  82°,  this 
correction  must  be  added  to  the  observed  altitude.     Tables  have 
been  computed,  from  which  we  may  obtain,  by  mere  inspection, 
the  correction  to  be  applied  to  the  observed  height  of  the  barom- 
eter.    See  Table  VIII.,  pages  258-259. 

14.  Correction  for  Capillary  Action. — By  capillary  action  the  col- 
umn of  mercury  in  the  tube  is  depressed  below  that  height  which 
would  just  balance  the  pressure  of  the  air,  and  a  correction  must 
be  added  to  the  observed  heights  of  the  barometer  in  order  to 
obtain  the  true  pressure  of  the  atmosphere.     This  correction  va- 
ries with  the  diameter  of  the  tube. 


CONSTITUTION   AND    WEIGHT   OF   THE   ATMOSPHERE. 


15 


'0.10  inch" 

'  0.140  inch. 

.20    " 

.058    " 

.30    " 

the  depression 

.029    " 

.40    " 

amounts  to 

.015    " 

.50    " 

.008    " 

.60    "     J 

.004    " 

In  a  tube  whose 
diameter  is 


15.  The  Aneroid  Barometer  is   an   instrument  for   measuring 

the  pressure  of  the  atmosphere  by 
means  of  the  elasticity  of  a  plate 
of  metal.  It  consists  of  a  cylin- 
drical brass  box,  about  three  inch- 
es in  diameter  and  half  an  inch  in 
height,  the  sides  of  which  are  made 
very  thin,  and  which  is  hermetic- 
ally sealed  after  the  air  has  been 
partly  exhausted  from  the  interior. 
When  the  pressure  of  the  atmos- 
phere increases,  the  inclosed  air  is 
compressed,  the  capacity  of  the  box 
is  diminished,  and  the  two  flat  ends 
approach  each  other.  When  the 
pressure  diminishes,  the  ends  resume  their  former  position,  in  con- 
sequence of  the  expansion  of  the  inclosed  air.  By  means  of  a 
combination  of  levers,  this  motion  of  the  ends  of  the  box  is  com- 
municated to  a  pointer,  which  travels  over  a  graduated  dial-plate, 
and  the  original  motion  is  magnified,  so  that  the  index  travels 
over  a  space  of  three  inches,  while  the  end  of  the  box  only  moves 
the  -g^-g-th  of  an  inch.  This  instrument  has  the  advantage  of  ex- 
treme portability,  and,  when  well  made,  will  measure  small  devi- 
ations from  the  mean  pressure  within  one  or  two  hundredths  of 
an  inch.  In  observations  requiring  great  accuracy,  it  should, 
however,  be  frequently  compared  with  a  standard  mercurial  ba- 
rometer. 

16.  Self-registering  Barometers. — In  order  to  diminish  the  labor 
of  frequent  observations  of  the  barometer,  attempts  have  been 
made  to  render  it  self- registering.  One  of  the  best  methods  of 
accomplishing  this  object  is  by  means  of  photography.  The  light 
of  a  lamp  or  gas-flame,  A,  is  concentrated  by  means  of  a  lens,  B, 


16  METEOROLOGY. 

Fig.  4. 


so  as  to  strike  upon  the  summit  of  the  column  of  mercury  in  the 
barometer  tube,  CD.  A  sheet  of  paper  suitably  prepared  for 
photographic  experiments  is  attached  to  a  frame,  F,  placed  behind 
a  screen,  G,  having  a  narrow  vertical  slit  placed  in  the  line  of  the 
rays  passing  through  B.  The  mercury  protects  a  portion  of  the 
paper  from  the  action  of  the  light  of  the  lamp,  while  above  the 
mercury  the  rays  of  the  lamp  fall  unobstructed  upon  the  paper. 
By  means  of  a  clock,  H,  the  paper  is  carried  steadily  forward  at 
the  rate  of  about  half  an  inch  per  hour,  and  thus  the  column  of 
mercury  leaves  upon  the  paper  a  permanent  record  of  its  height 
for  each  instant  of  the  day.  At  the  close  of  the  day  a  new 
rig.  s.  sheet  of  paper  must  be  ap- 

plied, and  thus  the  record  is 
continued.  Fig.  5  represents 
the  appearance  of  a  sheet  con- 
taining a  day's  observations, 
m't.  2h  4 6  8  10  noon  2h  4  6  8  io  m't.  A  graduation  upon  the  vert- 
ical side  of  the  sheet  indicates  differences  of  height,  while  a  grad- 
uation upon  the  horizontal  side  indicates  the  corresponding  hours 
of  observation. 

17.  Hardy's  Self-registering  Barometer  is  a  siphon  barometer, 
ABC,  both  ends  of  the  tube  having  the  same  diameter.  Upon 
the  surface  of  the  mercury  at  C  rests  an  iron  float,  to  which  is 
attached  a  cord  passing  over  a  pulley,  P,  and  from  the  other  end 
of  the  cord  is  suspended  a  counterpoise,  D  D.  The  float  is  thus 
made  to  rise  and  fall  with  the  mercury  in  the  barometer,  with- 
out interfering  with  the  free  motion  of  the  mercury,  and  this  mo- 


CONSTITUTION  AND   WEIGHT   OF   THE  ATMOSPHERE. 


17 


tion  is  copied  by  the  weight.  This  weight 
carries  a  pencil  whose  point  is  very  near  to 
a  large  vertical  cylinder,  E  E,  which  turns 
uniformly  about  its  axis.  This  cylinder, 
which  is  covered  with  a  sheet  of  paper,  is 
made  to  revolve  by  means  of  the  clock,  G. 
Every  half  hour  this  clock  moves  a  ham- 
mer, H  K,  whose  head  strikes  the  weight, 
D  D,  by  which  means  the  point  of  the  pen- 
cil is  pressed  against  the  cylinder,  and  makes 
a  mark  whose  position  indicates  the  height 
of  the  mercury  in  the  barometer.  On  the 
sheet  of  paper  is  traced  a  horizontal  line  di- 
vided into  equal  parts  to  indicate  the  hours 
of  the  day.  The  series  of  points  thus  mark- 
ed upon  the  paper  shows  the  movement  of  the  barometer  during 
24  hours. 

18.  Hough's  Printing  Barometer. — Mr.  G.  W.  Hough,  Director  of 
the  Dudley  Observatory  at  Albany,  has  invented  an  instrument 
which  furnishes  automatically  a  printed  record  of  the  pressure  of 
the  atmosphere  for  every  hour  of  the  day.  For  this  purpose  he 
employs  a  siphon  barometer,  and  a  float  resting  upon  the  mercu- 
ry in  the  open  arm.  This  float  supports  a  small  platinum  disk 
which  is  placed  horizontally  between  the  points  of  two  wires 
which  communicate  with  a  voltaic  battery.  These  wires  are  sup- 
ported by  a  screw,  S,  which  is  attached  to  a  toothed  wheel,  W. 
"When  the  mercury  rises  in  the  short  leg  of  the  siphon,  the  plati- 
num disk  is  raised,  and  touches  the  upper  wire,  closing  the  circuit 
through  an  electro-magnet,  advancing  the  wheel  W  one  tooth,  and 
raising  the  screw  S;  and  so  long  as  the  mercury  continues  to  rise, 
the  screw  S  rises  also.  When  the  mercury  in  the  siphon  falls, 
the  under  side  of  the  platinum  disk  is  brought  in  contact  with  the 
point  of  the  lower  wire,  closing  the  circuit  through  another  mag- 
net, moving  the  wheel  W  one  tooth  backward,  and  depressing  the 
screw  S.  Thus  the  screw  S  is  made  to  rise  or  fall  with  the  mer- 
cury in  the  barometer.  This  screw  carries  a  pencil,  which  traces 
upon  a  revolving  cylinder  a  line  showing  the  minutest  move- 
ments of  the  column  of  mercury  during  a  period  of  twenty-four 
hours.  This  same  screw  also  gives  motion  to  a  series  of  wheels 

B 


18  METEOKOLOGY. 

which  carry  types,  by  which  at  the  end  of  every  hour  the  height 
of  the  column  of  mercury  is  printed  on  a  slip  of  paper  to  the  ac- 
curacy of  the  thousandth  part  of  an  inch. 

19.  Mean  Height  of  the  Barometer. — If  we  record  the  height  of 
the  barometer  for  each  hour  of  the  day,  after  it  is  corrected  for 
temperature  and  capillarity,  and  divide  the  sum  of  the  results  by 
24,  we  obtain  the  mean  height  for  the  day.     If  we  divide  the  sum 
of  the  daily  means  for  a  month  by  the  number  of  days,  we  ob- 
tain the  mean  height  for  the  month.     If  we  divide  the  sum  of  the 
monthly  means  by  12,  we  obtain  the  mean  height  for  the  year.     If 
we  divide  the  sum  of  the  annual  means  for  a  long  period  by  the 
number  of  years,  we  obtain  the  mean  height  of  the  barometer  for 
the  place  of  observation.     The  mean  height  of  the  barometer  at 
New  York  (reduced  to  the  level  of  the  sea)  is  30.036  inches. 

V  t/1 

20.  Distribution  of  Atmospheric  Pressure  over  the  Globe. — In  the 
neighborhood  of  the  equator,  the  mean  pressure  of  the  atmosphere 
(reduced  to  the  level  of  the  sea)  is  everywhere  somewhat  less 
than  30  inches.     In  most  places  it  is  as  low  as  29.9  inches,  and 
in  portions  of  Africa  and  India  it  is  a  little  below  29.8  inches. 
As  we  advance  northward  the  mean  pressure  increases,  and  there 
is  a  belt  surrounding  the  globe  whose  average  breadth  is  about 
25°,  within  which  the  mean  pressure  everywhere  exceeds  30 
inches.     This  belt  includes  two  centres  of  maximum  pressure — 
one  in  the  eastern  part  of  the  Atlantic  Ocean  near  latitude  32°, 
where  the  mean  pressure  exceeds  30.2  inches,  the  other  is  in  the 
eastern  part  of  the  Pacific  Ocean  near  latitude  30°,  where  the 
mean  pressure  exceeds  30.1  inches.     North  of  this  belt  of  high 
pressure,  the  mean  pressure  is  everywhere  less  than  30  inches; 
and  there  are  two  centres  of  minimum  pressure — one  near  Iceland, 
where  the  mean  pressure  is  a  little  less  than  29.6  inches,  and  the 
other  near  the  Aleutian  Islands,  where  the  mean  pressure  is  a 
little  less  than  29.7  inches. 

In  the  southern  hemisphere  there  is  also  a  belt  surrounding 
the  globe  within  which  the  mean  pressure  is  above  30  inches, 
and  in  portions  of  the  South  Atlantic  Ocean  the  .mean  pressure 
somewhat  exceeds  30.1  inches.  The  pressure  is  generally  great- 
est near  the  parallel  of  25°.  South  of  this  belt  the  mean  pressure 
rapidly  diminishes,  and  near  latitude  70°  the  mean  pressure  has 


CONSTITUTION   AND  WEIGHT  OF  THE  ATMOSPHERE. 


19 


Fig.  S. 


been  found  to  be  only  28.88  inches.  The  cause  of  this  unequal 
distribution  of  atmospheric  pressure  will  be  considered  here- 
after. 

21.  Inequality  of  the  Monthly  Means. — The  mean  height  of  the 
barometer  is  not  the  same  for  each  month  of  the  year,  being  gen- 
erally less  in  summer  than  in  winter.     At  many  places  the  ine- 
quality amounts  to  half  an  inch,  while  at  other  places  it  almost 
entirely  disappears.     At  Pekin,  in  China,  the  mean  height  of  the 
barometer  is  least  in  July,  from  which  time  the  mean  pressure 
increases  uninterruptedly  to  January,  after  which  it  declines  un- 
interruptedly to  the  next  July  ;  the  pressure  in  January  exceed' 
ing  that  in  July  by  three  fourths  of  an  inch.     A  similar  law  pre- 
vails throughout  a  considerable  portion  of  the  continent  of  Asia. 
The  cause  of  this  fluctuation  will  be  explained  on  page  63. 

In  the  middle  latitudes  of  Europe  and  America,  the  mean  height 
of  the  barometer  is  usually  about  the  same  for  each  month  of  the 

year.  At  Boston  there  are  no  two 
months  whose  mean  pressures  differ 
by  more  than  one  tenth  of  an  inch. 
A  similar  remark  is  applicable  to 
London  and  Paris.  These  varia- 
tions of  pressure  are  conveniently 
represented  by  means  of  curve  lines. 
We  draw  upon  a  sheet  of  paper  a 
horizontal  line  J  J,  and  divide  it 
into  twelve  equal  parts  to  represent 
the  different  months  of  the  year, 
A  s  o  N  D  J  an(j  through  these  points  of  divis- 
ion we  draw  a  system  of  vertical  lines.  Upon  each  of  the  verti- 
cal lines  we  set  off  the  mean  height  of  the  barometer  for  the  cor- 
responding month,  and  connect  all  these  points  by  a  broken  line. 
We  thus  obtain  a  line  whose  curvature  represents  the  mean  mo- 
tion of  the  barometer  for  each  month  of  the  year.  The  four 
curves  of  Fig.  8  show  the  motion  of  the  barometer :  P  for  Pekin, 
H  for  Havana,  L  for  London,  and  B  for  Boston.  See  Table  X. 

22.  Hourly  Variations. — If  we  record  the  height  of  the  barom- 
eter for  each  hour  of  the  day,  during  a  long  period  of  time,  and 
take  the  mean  of  all  the  observations  for  each  hour,  we  shall  find 


J  F  M  A  M  J 


20 


METEOROLOGY. 


Fig.  9. 


that  these  averages  are  not  equal  to  each  other.  The  height  of 
the  barometer  is  greatest  about  10  A.M.,  and  least  about  4  P.M. 
Smaller  fluctuations  are  also  observed  during  the  night,  the  ba- 
rometer attaining  a  second  maximum  about  10  P.M.,  and  a  sec- 
ond minimum  about  4  A.M.  The  amount  of  this  diurnal  oscilla- 
tion is  greatest  at  the  equator,  where  its  value  is  0.104  inch,  and 
it  diminishes  as  we  proceed  toward  either  pole.  In  latitude  40° 
it  is  reduced  to  0.05  inch ;  and  in  latitude  70°  it  is  only  0.003 
inch.  On  elevated  mountains  the  diurnal  oscillation  is  less  than 
it  is  at  the  level  of  the  sea,  and  the  maximum  and  minimum 
pressures  occur  at  a  later  hour. 

These  variations  of  pressure  may  be  represented  by  curve  lines. 
We  draw  upon  a  sheet  of  paper  several  vertical  and  equidistant 

lines  to  represent  the  hours  of  the  day. 
We  set  off  upon  each  of  the  vertical 
lines  the  mean  height  of  the  barome- 
ter for  the  corresponding  hour,  and 
connect  all  these  points  by  a  broken 
line.  We  thus  obtain  a  line  whose 
curvature  represents  the  mean  motion 
of  the  barometer  for  each  hour  of  the 
day.  The  three  curves  of  Fig.  9  show  the  motion  of  the  barome- 
ter, E  for  the  equator,  P  for  Philadelphia,  and  S  for  St.  Peters- 
burg. These  curves  arc  seen  to  have  two  daily  maxima  and  two 
daily  minima.  See  Table  XI. 

23.  Inequality  depending  on  the  Position  of  the  Moon. — There  is 
a  small  fluctuation  in  the  pressure  of  the  atmosphere  depending 
on  the  position  of  the  moon ;  but  this  variation  is  exceedingly 
minute,  and  can  only  be  detected  by  taking  the  mean  of  the  most 
accurate  observations  continued  for  a  long  period  of  time.  At 
Singapore,  latitude  1°  18',  when  the  moon  is  on  the  meridian,  the 
pressure  of  the  atmosphere  is  0.0057  inch  greater  than  when  the 
moon  is  six  hours  from  the  meridian.  At  St.  Helena,  latitude 
15°  55',  when  the  moon  is  on  the  meridian,  the  pressure  of  the 
atmosphere  is  0.004  inch  greater  than  when  the  moon  is  six 
hours  from  the  meridian.  In  higher  latitudes  the  difference  of 
pressure  is  still  less.  These  results  indicate  a  feeble  tide  in  OUT 
atmosphere,  similar  to  the  tides  of  the  ocean. 


9    noon    3 


9  m't 


CONSTITUTION  AND   WEIGHT  OF  THE   ATMOSPHERE.          21 

24.  Accidental  Variations. — The  non-periodic  oscillations  of  the 
barometer  are  far  greater  than  the  periodical  ones.     In  the  mid- 
dle latitudes  the  barometer  is  almost  constantly  in  motion,  and 
the  fluctuations  are  so  great  and  so  irregular  that  the  periodical 
movements  are  only  detected  by  taking  the  mean  of  a  long  series 
of  observations.     The  difference  between  the  greatest  and  least 
heights  of  the  barometer  during  a  single  month  is  called  the 
monthly  oscillation ;   and  by  combining  observations  extending 
over  a  great  number  of  years,  we  obtain  the  mean  monthly  oscil- 
lation.    The  mean  monthly  oscillation  is  least  in  the  neighbor- 
hood of  the  equator,  and  increases  as  we  approach  the  poles.    At 
the  equator  it  is  but  little  over  one  tenth  of  an  inch ;  in  latitude 
30°  it  is  four  tenths  of  an  inch ;  in  latitude  45°,  over  the  Atlantic 
Ocean,  it  is  one  inch ;  in  latitude  65°  it  is  one  inch  and  a  third ; 
and  in  latitude  78°  it  is  one  inch  and  a  fifth.     During  the  three 
winter  months,  the  mean  monthly  oscillation  is  about  one  third 
greater  than  the  numbers  here  stated.     Ovei  2he  continents  of 
Europe  and  America,  the  oscillations  are  generally  less  than  over 
the  Atlantic  on  the  same  parallel. 

25.  Extreme  Fluctuations  of  the  Barometer. — The  exl?eme  fl  actu- 
ations of  the  barometer  are  much  greater  than  the  numbers  here 
given.     The  greatest  height  which  the  barometer  at  Boston  has 
attained  in  37  years  is   31.125  inches,  and  the  least  is  28.47 
inches ;  the  difference  being  2.655  inches,  or  -^th  of  the  aver- 
age height  of  the  column.     At  London,  the  greatest  observed 
range  of  the  barometer  is  three  inches,  while  at  St.  Petersburg 
and  in  Iceland  it  is  3.5  inches.     At  Christiansborg,  near  the  equa- 
tor, the  entire  range  of  the  barometer  in  five  years  was  0.47  inch. 

26.  Influence  of  the  Wind. — The  height  of  the  barometer  is  sensi- 
bly influenced  by  the  direction  of  the  wind.     At  Philadelphia  the 
barometer  generally  stands  highest  when  the  wind  is  northeast, 
and  lowest  when  the  wind  is  west  or  southwest,  the  mean  differ- 
ence in  -the  height  of  the  barometer  for  these  different  winds  being 
a  quarter  of  an  inch.     Throughout  the  northwest  part  of  Europe 
the  barometer  stands  highest  when  the  wind  is  northeast,  and 
lowest  when  the  wind  is  south,  the  mean  difference  for  these  two 
winds  being  0.22  inch. 


22  METEOKOLOGY. 

27.  Pressure  affected  by  Height  of  Station. — When  a  barometer 
is  elevated  above  the  surface  of  the  earth,  the  column  of  mercury 
sinks,  because  the  force  which  sustains  the  column,  that  is,  the 
weight  of  the  superincumbent  air,  is  diminished.     By  comparing 
the  height  of  the  mercury  in  barometers  at  two  stations,  one  of 
which  is  above  the  other,  we  ascertain  the  weight  of  a  column  of 
air  extending  from  the  lower  to  the  higher  station.     For  exam- 
ple, if  the  mercury  in  the  lower  barometer  stands  at  30  inches, 
and  in  the  higher  barometer  at  29  inches,  it  follows  that  a  column 
of  air  extending  from  the  lower  to  the  higher  station  has  a  weight 
equal  to  that  of  a  column  of  mercury  one  inch  high.     Now  the 
density  of  mercury  is  10,464  times  that  of  air ;  hence  a  fall  of 
one  inch  in  the  barometer  would  indicate  an  elevation  of  10,464 
inches,  or  872  feet,  above  the  first  station,  provided  the  density  of 
the  air  were  the  same  at  both  stations. 

28.  Heights  measured  by  Barometer. — Since  the  air  is  readily 
compressed,  its  density  rapidly  diminishes  as  the  height  increases. 
Mathematicians  have  endeavored  to  discover  the  exact  relation 
between  the  change  of  barometric  heights  and  the  difference  of 
level  of  the  two  stations   of  observation.      Laplace  deduced  a 
formula  which  is  designed  to  take  account  of  all  the  corrections 
required  for  attaining  the  greatest  accuracy,  such  as  the  change 
of  temperature  of  the  air  between  the  two  stations,  the  diminu- 
tion of  gravity  on  a  vertical  line,  etc.    According  to  this  formula, 
in  the  neighborhood  of  New  York,  when  the  atmosphere  is  at  its 
mean  state,  if  we  ascend  above  the  level  of  the  sea 

917  feet,  the  barometer  sinks  1  inch. 
1860    "  "  "       2  inches. 

2830    "  "  "       3     " 

3830    "  "  "      4     " 

4861    "  "  "      5     " 

Table  IX,  page  260,  affords  the  means  of  determining  the  dif- 
ference in  the  heights  of  any  two  places  by  means  of  barometric 
observations. 


TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH.     23 


CHAPTER  II. 

TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH. 

29.  Climatology. — Climatology  is  the  science  of  climates.     By 
the  climate  of  a  country  we  understand  its  condition  relative  to 
all  those  atmospheric  phenomena  which  influence  organized  be- 
ings.    Climate  depends  upon  the  mean  temperature  of  the  year; 
upon  that  of  each  month  and  each  day ;  upon  the  maximum  and 
minimum  temperatures;  upon  the  frequency  and  suddenness  of 
the  atmospheric  changes ;   upon  the  transparency  of  the  atmos- 
phere and  the  amount  of  solar  radiation ;  upon  the  moisture  of 
the  air  and  the  earth  ;  upon  the  prevalence  of  fogs  and  dew ;  the 
amount  of  rain  and  snow ;  the  frequency  of  thunder-storms  and 
hail ;   the  direction,  force,  and  dry  ness  of  the  winds,  etc.     All 
these  particulars  can  only  be  determined  by  long  and  careful  ob- 
servations. 

30.  Thermometer. — The  changes  of  temperature  of  the  air  are 
measured  by  means  of  the  thermometer.     This  instrument  gen- 
erally consists  of  a  small  glass  bulb,  to  which  is  attached  a  long 
glass  tube,  having  a  very  small  bore.     The  bulb  is  filled  with 
mercury  or  alcohol,  which  also  rises  somewhat  within  the  tube. 
Now  mercury  and  alcohol  are  very  much  expanded  by  an  in- 
crease of  heat,  while  glass  expands  very  little.     If,  then,  the  tem- 
perature of  the  thermometer  rises,  since  the  mercury  expands 
more  than  the  bulb  which  contains  it,  it  overflows  the  bulb,  and 
is  forced  up  into  the  small  tube.     If  the  temperature  falls,  the 
mercury  contracts  more  than  the  glass  bulb,  and  the  mercury  in 
the  tube  descends  to  fill  the  vacuum  created  in  the  bulb.     Thus 
the  changes  of  temperature  to  which  the  thermometer  is  subject- 
ed are  indicated  by  the  ascent  or  descent  of  the  mercury  in  the 
small  tube. 

31.  Graduation  of  the  Scale. — In  order  that  we  may  have  an  in- 
telligible measure  of  these  changes  of  temperature,  the  tube  must 


24  METEOROLOGY. 

be  graduated  according  to  some  general  principles.  We  need 
two  invariable  temperatures  for  the  determination  of  two  fixed 
points  upon  the  scale.  The  temperatures  generally  adopted  for 
this  purpose  are  those  of  melting  ice  and  boiling  water;  and  the 
interval  between  these  points  is  variously  divided  in  different 
countries.  Upon  Fahrenheit's  thermometer,  melting  ice  is  mark- 
ed 32°,  and  boiling  water  212°,  the  interval  being  divided  into 
180  equal  parts.  The  same  graduation  is  extended  downward 
from  32°  to  zero,  and  may  be  continued  below  zero  as  far  as  is 
desired.  Degrees  below  zero  are  distinguished  by  the  minus  sign. 
Thus  we  may  have  a  temperature  of  40°  above  zero,  or  40°  be- 
low zero.  Fahrenheit's  scale  is  generally  used  in  England  and 
the  United  States. 

Upon  the  Centigrade  thermometer,  the  freezing  point  is  mark- 
ed 0,  and  the  boiling  point  100.  This  thermometer  is  generally 
used  in  France.  Upon  Reaumur's  thermometer,  the  freezing 
point  is  marked  0,  and  the  boiling  point  80.  This  thermometer 
is  generally  used  in  Germany  and  Russia. 

32.  Requisites  of  a  good  Thermometer. — It  is  evident  that  the  de- 
grees upon  the  thermometer  scale  should  correspond  to  equal  vol- 
umes of  mercury.  If  the  tube  of  the  thermometer  were  through- 
out of  uniform  bore,  then  the  divisions  upon  the  scale  should  be 
throughout  of  equal  length  ;  but  if  the  tube  be  not  of  uniform 
bore,  these  equal  volumes  will  correspond  to  unequal  lengths 
upon  different  parts  of  the  scale.  Now  it  is  impossible  to  obtain 
a  glass  tube  perfectly  cylindrical,  and  therefore  when  an  accurate 
graduation  is  required,  we  proceed  as  follows:  Having  selected  a 
tube  whose  bore  is  as  nearly  uniform  as  possible,  \ve  introduce 
rig.  10.  into  it  a  short  column  of  mercury 

l~         i           i           i  '-    A  B.  and  mark  its  extremities  upon 

A  B  C  D 

the  tube.  Then,  by  agitating  the  tube, 

we  push  the  mercury  along  to  B  C,  so  that  its  left  extremity  may 
occupy  the  same  position  as  the  right  extremity  in  the  first  trial ; 
and  mark  the  extremity  C.  The  volumes  of  A  B  and  B  C  are 
evidently  equal.  We  thus  crowd  the  column  of  mercury  along 
from  one  end  of  the  tube  to  the  other,  and  divide  it  into  portions 
of  equal  volume.  Each  of  these  portions,  A  B,  B  C,  C  D,  etc., 
should  then  be  made  to  contain  the  same  number  of  divisions  of 
the  scale. 


TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH.     25 

When  a  standard  thermometer  has  been  constructed  in  this 
manner,  other  thermometers  are  frequently  graduated  by  com- 
parison with  it  at  several  different  points  of  the  scale. 

33.  Self-registering  Thermometers. — It  is  frequently  desired  t6 
determine  the  greatest  heat  or  the  greatest  cold  experienced  dur- 
ing a  day  or  some  longer  interval  of  time.  To  do  this  with  an 
ordinary  thermometer,  it  is  necessary  that  the  instrument  be  fre- 
quently observed  at  short  intervals.  Such  observations  are  very 
laborious ;  and  in  order  to  diminish  this  labor,  self-registering  ther- 
mometers have  been  invented. 

The  following  is  one  form  of  a  thermometer  for  registering  the 
highest  temperature.  A  small  piece  of  steel  wire  c,  about  half  an 
inch  in  length,  and  finer  than  the  bore  of  the  thermometer,  is  in- 
troduced into  the  tube  of  a  mercurial  thermometer  above  the 
mercury.  The  thermometer  is  placed  with  its  stem  A  B  in  a 

Fig.  11. 


horizontal  position,  and  the  steel  index  is  brought  into  contact 
with  the  extremity  of  the  column  of  mercury.  Now,  when  the 
heat  increases  and  the  mercury  expands,  the  index  c  will  be 
thrust  forward ;  but  when  the  temperature  falls,  and  the  mercury 
contracts,  the  index  will  be  left  behind.  The  point  of  the  scale 
where  the  index  is  found  shows  therefore  the  greatest  degree  of 
heat  to  which  the  instrument  has  been  subjected  since  the  last 
observation. 

34.  Minimum  Thermometer. — The  lowest  degree  to  which  the 
thermometer  has  fallen  may  be  indicated  as  follows :  A  spirit 
thermometer  is  placed  with  its  stem  D  E  horizontal,  and  within 
the  tube  is  a  very  fine  glass  rod,  or  index,  n,  about  half  an  inch 
in  length,  and  a  little  smaller  than  the  bore  of  the  tube.  This 
index  is  immersed  in  the  column  of  alcohol,  but  must  be  brought 
into  contact  with  the  extremity  of  the  column.  On  account  of 
the  capillary  adhesion  between  the  alcohol  and  the  glass,  when 
the  alcohol  contracts,  it  drags  along  with  it  the  glass  index;  but 


26  METEOROLOGY. 

when  the  alcohol  expands,  it  passes  by  the  index  without  dis- 
placing it,  so  that  the  position  of  the  index  shows  the  lowest  tem- 
perature to  which  the  instrument  has  been  subjected  since  the 
last  observation. 

These  instruments  are  especially  adapted  to  indicate  the  maxi- 
mum and  the  minimum  temperature  in  twenty-four  hours.  The 
steel  index  being  placed  in  contact  with  the  mercury,  and  one 
extremity  of  the  glass  index  being  made  to  coincide  with  the  ex- 
tremity of  the  column  of  alcohol,  the  position  of  the  two  indices 
on  the  following  day  will  show  what  has  been  the  highest  and 
what  has  been  the  lowest  temperature  during  the  last  twenty- 
four  hours.. 

35.  Phillips's  Maximum  Thermometer. — In  this  thermometer  a 
small  portion  of  the  column  of  mercury  is  separated  from  the  re- 
mainder of  the  column  by  an  extremely  minute  speck  of  air,  so 
that  this  detached  column  serves  the  same  purpose  as  the  steel 
wire  in  the  ordinary  maximum  thermometer.     The  end  of  this 
detached  column  remains  at  the  point  of  maximum  temperature, 
while  the  other  part  of  the  column  retreats  toward  the  bulb  when 
the  temperature  declines.     By  bringing  the  instrument  to  a  verti- 
cal position  with  the  bulb  downward,  the  detached  portion  de- 
scends nearly  into  contact  with  the  remainder  of  the  column,  but 
is  prevented  from  uniting  with  it  by  the  presence  of  the  air  speck. 
This  instrument  is  susceptible  of  very  great  precision. 

36.  Photographic  Register  of  the  Thermometer. — In  some  observa- 
tories, the  height  of  the  thermometer  is  registered  photographic- 
ally, in  a  manner  similar  to  that  described  in  Art.  16.     The  light 
of  a  lamp  is  concentrated  by  means  of  a  lens,  so  as  to  strike  upon 
the  summit  of  the  column  of  mercury  in  the  thermometer.     A 
sheet  of  paper  suitably  prepared  for  photographic  experiments 
is  placed  behind  the  thermometer,  and  receives  the  shadow  cast 
by  the  mercury.     By  means  of  clock-work,  the  paper  is  carried 
steadily  forward,  and  thus  the  column  of  mercury  leaves  upon 
the  paper  a  record  of  its  height  at  each  instant  of  the  twenty-four 
hours.      This  is  in  some  respects  the  best  self-registering  ther- 
mometer known,  although  the  record  is  usually  not  very  sharp, 
and  therefore  not  as  accurate  as  could  be  desired. 


TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH.     27 

37.  Cause  of  the  variations  of  Temperature. — The  sun  is  the  prin- 
cipal cause  of  the  variations  of  the  temperature  of  the  atmosphere. 
The  amount  of  heat  which  the  sun  communicates  in  a  given  time 
depends  upon  the  elevation  of  the  sun  above  the  horizon,  and 
upon  the  transparency  of  the  atmosphere..    The  difference  be- 
tween summer  and  winter  depends  upon  the  time  that  the  sun 
remains  above  the  horizon,  as  well  as  upon  its  distance  from  the 
zenith  of  the  observer. 

38.  How  the  Atmosphere  is  Heated. — The  atmosphere  is  heated 
in  three  ways :  by  the  direct  rays  of  the  sun ;  by  contact  with 
the  warmer  earth ;  and  by  the  radiation  and  reflection  of  heat 
from  the  earth. 

A  portion  of  the  rays  of  heat  which  are  emitted  by  the  sun 
are  absorbed  by  our  atmosphere  before  they  can  reach  the  earth's 
surface.  It  is  estimated  that  in  clear  weather  the  atmosphere 
absorbs  about  one  fourth  of  the  rays  which  traverse  the  atmos- 
phere vertically.  The  remaining  rays  are  received  upon  the 
earth's  surface,  by  which  means  the  earth  is  heated.  This  heat 
is  thence  communicated  to  the  air  which  rests  upon  the  earth ; 
and  this  air,  being  thereby  rendered  lighter,  rises  and  gives  place 
to  colder  air  from  above.  This  in  turn,  by  contact  with  the  earth, 
becomes  heated,  and  rises,  and  thus  there  is  maintained  a  con- 
tinued circulation  between  the  strata  of  air  in  the  neighborhood 
of  the  earth. 

A  portion  of  the  heat  which  the  earth  receives  from  the  sun 
radiates  into  space.  These  rays  are  partly  absorbed  by  the  air, 
especially  by  its  lower  strata,  and  these  strata,  in  their  turn,  diffuse 
invisible  rays  of  heat  in  every  direction. 

The  effect  of  the  direct  rays  of  the  sun  is  plainly  seen  in  win- 
ter when  the  ground  is  covered  with  snow.  In  the  vicinity  of 
trees  and  posts  the  snow  disappears  more  rapidly  than  it  does 
where  the  surface  of  the  snow  is  entirely  unbroken.  This  is  be- 
cause the  rays  of  the  sun  are  absorbed  by  the  dark  surface  of  the 
trees  more  readily  than  they  are  by  the  snow.  Thus  the  trees 
are  warmed,  and  these,  in  their  turn,  send  out  rays  of  heat  by 
which  the  adjacent  snow  is  melted. 

39.  Proper  Exposure  of  a  Thermometer. — For  the  purpose  of 
measuring  the  temperature  of  the  air,  a  thermometer  should  be 


28  METEOROLOGY. 

exposed  in  the  open  air,  where  the  circulation  is  unobstructed. 
It  should  face  the  north,  and  should  be  always  in  the  shade.  It 
should  be  removed  at  least  a  foot  from  the  wall  of  the  building, 
and  should  be  elevated  about  ten  feet  from  the  ground.  It  should 
be  protected  against  .the  heat  reflected  by  neighboring  objects, 
such  as  buildings  or  a  sandy  soil,  and  it  should  be  sheltered  from 
the  rain.  If  the  thermometer  should  happen  to  become  moisten- 
ed by  rain,  the  bulb  should  be  carefully  dried  about  five  minutes 
before  making  the  observation  ;  since  drops  of  water,  by  their 
evaporation,  would  lower  the  temperature  of  the  mercury  in  the 
bulb. 

In  order  to  secure  all  these  advantages,  it  is  generally  found 
necessary  to  cover  the  thermometer  with  a  wooden  frame  of  open 
lattice- work ;  but  this  covering  should  be  such  as  to  allow  a  per- 
fectly free  circulation  of  air  about  the  thermometer,  and  it  should 
be  such  as  readily  to  acquire  the  temper- 
ature of  the  surrounding  air. 

Fig.  12  represents  a  frame  adopted  at 
Greenwich  Observatory  for  supporting  the 
thermometers.  It  consists  of  two  paral- 
lel inclined  boards,  with  a  small  projecting 
roof,  beneath  which  the  thermometers  are 
suspended,  so  that  the  air  circulates  free- 
ly about  the  bulbs.  The  whole  frame  re- 
volves on  an  upright  post,  and  the  inclined 
side  is  always  turned  toward  the  sun. 

40.  Hourly  Observations  of  the  Thermome- 
ter.— In  order  to  determine  the  laws  which  govern  the  variations 
of  the  temperature  of  the  atmosphere,  we  require  that  observ- 
ations should  be  made  from  hour  to  hour,  both  night  and  day, 
throughout  a  period  of  several  years.  Such  observations  have 
been  made  at  many  different  places.  The  most  extensive  series 
.of  this  kind  in  North  America  was  made  at  Toronto,  where  bi- 
hourly  observations  were  continued  for  ten  years.  At  Philadel- 
phia, hourly  observations  were  made  for  two  and  a  half  years, 
and  bi-hourly  observations  for  another  two  and  a  half  years.  At 
Washington,  observations  every  two  hours  were  continued  for 
two  and  a  half  years.  Similar  observations  upon  a  less  extensive 
scale  have  been  made  at  a  few  other  places  in  this  country. 


TEMPERATUEE   OF  THE   AIR  AND   OF  THE   EARTH. 


29 


Fig.  13. 


53 
56 
54 
52 
50 
4S 
46 
44 
42 

^e£t 

—  ^ 

/ 

x 

x 

/ 

V 

/ 

\ 

/ 

\ 

} 

\ 

-^ 

~^ 

^-^ 

£ 

41.  Hourly  Variations  of  Temperature. — The  temperature  of  a 
place  changes  from  one  hour  to  another,  according  to  the  distance 
of  the   sun  from  the  horizon.     If  we  take  the   average   of  all 

the  temperatures  observed 
at  each  houT  of  the  day  for 
a  long  period  of  time,  we 
shall  find  that  the  mean 
hourly  variations  of  temper- 
ature are  extremely  regular. 
Figure  13  shows  the  general 

'm^sh.  4  e  s"  "wToon"2h7^  e  8  10  m't  ]aw  of  the  change  of  tem- 
perature  at  New  Haven.  The  abscissas  represent  the  hours  of 
the  day,  and  the  ordinates  the  temperatures  observed. 

We  see  that  on  each  day  there  is  a  maximum  and  minimum 
of  temperature.  At  New  Haven,  the  minimum  occurs  about  an 
hour  before  the  rising  of  the  sun,  and  the  maximum  about  two 
hours  after  noon.  In  the  average  of  the  entire  year,  the  temper- 
ature is  increasing  during  nine  hours  of  the  day,  and  decreasing 
during  the  remaining  fifteen  hours  of  the  day. 

The  highest  temperature  of  the  day  should  occur  when  the 
amount  of  heat  lost  each  instant  by  radiation  is  just  equal  to  the 
heat  received  from  the  sun.  Before  midday,  the  earth  receives 
from  the  sun  more  heat  than  it  loses  by  radiation,  and  its  temper- 
ature rises.  After  noon,  the  earth,  receives  each  instant  from  the 
sun  less  heat  than  it  did  at  noon ;  but  the  heat  received  is  still 
greater  than  that  which  is  lost  by  radiation.  Hence  the  maxi- 
mum takes  place  some  time  after  noon.  During  the  night  we  re- 
ceive no  direct  heat  from  the  sun,  and  the  earth  cools  by  radia- 
tion. The  lowest  temperature  should  occur  when  the  heat  re- 
ceived each  instant  from  the  returning  sun  is  just  equal  to  the 
loss  by  radiation.  This  occurs  about  an  hour  before  sunrise. 

42.  Mean  Temperature  of  a  Day. — The  mean  temperature  of  a 
day  is  the  mean  of  the  twenty-four  observations  taken  at  each 
hour  of  the  day.     Since  hourly  observations  of  the  thermometer 
are  very  laborious,  it  is  important  to  discover  simpler  methods  of 
ascertaining  the  mean  daily  temperature.     The  following  are  the 
principal  methods  which  have  been  employed  for  this  purpose. 


43.   From  the  Maximum   and  Minimum  Temperatures.  —  The 


30  METEOROLOGY. 

mean  of  the  highest  and  lowest  degrees  of  the  thermometer  dur- 
in»  the  twenty-four  hours  differs  but  little  from  the  mean  derived 
from  hourly  observations ;  and  when  self-registering  thermome- 
ters can  be  procured,  this  is  a  convenient  mode  of  obtaining  the 
mean  daily  temperature.  This  method  is  not,  however,  entirely 
accurate,  since  the  mean  of  the  two  daily  extremes  is  generally  a 
little  greater  than  the  mean  for  the  twenty-four  hours.  At  New 
Haven  the  average  difference  of  the  two  results  for  the  entire 
year  is  about  half  a  degree ;  being  nearly  an  entire  degree  in 
winter,  and  about  zero  in  summer.  When  the  highest  accuracy 
is  required,  a  small  correction  should  therefore  be  applied  for  the 
error  of  this  method. 

44.  From  Observations  at  a  single  Hour. — "When  self-registering 
thermometers  can  not  be  obtained,  one  of  the  following  methods 
may  be  practiced :  Twice  during  each  day  the  height  of  the  ther- 
mometer must  coincide  with  the  mean  temperature  of  the  day. 
At  New  Haven,  this  coincidence  occurs  about  a  quarter  before 
nine  in  the  morning,  and  also  about  a  quarter  before  eight  in  the 
evening.     We  should,  therefore,  obtain  very  nearly  the  mean 
temperature  by  a  single  daily  observation  at  either  of  these  hours. 
Since,  however,  at  these  times,  the  changes  of  temperature  are 
quite  rapid,  a  considerable  error  would  result  if  the  observation 
were  made  a  little  too  soon  or  a  little  too  late.     Moreover,  these 
hours  vary  at  different  localities,  and  they  also  vary  with  the  sea- 
son of  the  year,  so  that  it  is  better  to  deduce  the  mean  tempera- 
ture from  two  or  more  daily  observations. 

45.  From  two  Hours  of  the  same  Name. — It  is  found  that  the 
mean  temperature  of  any  two  hours  of  the  same  name  differs  but 
little  from  the  mean  of  the  twenty-four  hours.     Thus  the  mean 
of  two  observations  at  6  A.M.  and  6  P.M.,  is  nearly  the  same  as 
that  of  two  observations  at  7  A.M.  and  7  P.M.,  or  8  A.M.  and  8 
P.M.,  etc. ;  and  at  New  Haven  the  mean  of  two  observations  at 
10  A.M.  and  10  P.M.  differs  only  about  one  third  of  a  degree 
from  the  mean  of  the  twenty-four  hours.     These  hours  (10  A.M. 
and  P.M.)  are  better  than  any  other  two  hours  for  furnishing  the 
mean  temperature ;  and  the  mean  of  these  hours  is  generally 
nearer  the  mean  temperature  of  the  day  than  the  mean  of  the 
two  daily  extremes. 


TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH. 


31 


46.  From  three  Daily  Observations. — A  still  more  reliable  result 
may  be  derived  from  three  daily  observations.     The  mean  of  ob- 
servations at  6  A.M.,  2  and  9  P.M.,  gives  very  nearly  the  mean 
temperature  of  the  day.     The  mean  of  observations  at  7  A.M., 
2  and  9  P.M.,  is  a  little  too  great;  but  if  we  add  twice  the  nine 
o'clock  observation  to  the  sum  of  the  other  two  observations,  and 
divide  the  result  by  four,  the  error  of  the  result  for  the  separate 
months  at  New  Haven  in  only  one  instance  exceeds  a  quarter 
of  a  degree,  and  for  the  entire  year  differs  but  one  hundredth  of 
a  degree  from  the  true  mean  temperature.     It  is  found  that  for 
nearly  every  variety  of  climate  this  method  furnishes  the  best 
result  which  can  be  deduced  from  any  three  daily  observations, 
and  these  are  therefore  the  three  hours  to  be  generally  recom 
mended  to  observers.    See  Tables  XV.  and  XVI. 

47.  Mean  lemperature  of  the  Months. — The  mean  temperature 

Fig.  14.  of  a  month  is  found  by 

dividing  the  sum  of  the 
daily  means  by  the  num- 
ber of  days.  Figure  14 
shows  the  mean  temper- 
ature of  each  month  of 
the  year  at  New  Haven, 
and  also  the  mean  maxi- 
mum and  minimum  for 
the  month,  according  to 
86  years  of  observations. 
The  months  are  arranged 
upon  the  horizontal  line, 
and  the  temperature  for 
each  month  is  represented  by  the  corresponding  ordinate.  The 
upper  and  lower  curves  pass  through  the  maxima  and  minima 
temperatures  for  the  different  months,  and  the  intermediate  curve 
corresponds  to  the  monthly  mean  temperature. 

We  find  that  at  New  Haven,  1st.  The  warmest  months  of  the 
year  are  July  and  August,  and  the  maximum  for  the  year  occurs 
near  July  24th.  2d.  The  coldest  month  of  the  year  is  January, 
and  the  minimum  for  the  year  occurs  near  January  21st.  3d.  The 
difference  between  the  minimum  and  maximum  for  each  month 
is  greater  in  the  cold  months  than  in  the  warm  months.  4th.  The 


90° 
SO 
TO 

40 
SO 

0 

/*, 

tXIMl 

M    "-• 

V. 

/ 

\ 

/ 

\ 

/ 

^  

\ 

/ 

A 

'ME» 

M      "* 

V 

\ 

2 

/ 

\ 

\ 

/ 

/ 

' 

' 

^ 

/ 

^ 

X 

\ 

s 

/ 

/ 

MINI 

HUH\ 

\1 

/ 

/ 

\ 

\ 

/ 

/ 

\ 

s 

/ 

/ 

> 

. 

\ 

^ 

^ 

/ 

s 

N- 

/ 

\ 

' 

\ 

/ 

* 

. 

/ 

\ 

y 

s 

F   M   A   M   J 


32  METEOROLOGY. 

mean  temperature  of  the  month  of  April  is  two  degrees  below  the 
mean  temperature  of  the  year,  while  that  of  October  is  two  de- 
grees above  the  mean  for  the  year ;  and  the  mean  temperature  of 
the  two  months  April  and  October  differs  less  than  one  tenth  of 
a  degree  from  the  mean  temperature  of  the  year. 

48.  Monthly  Change  in  different  Latitudes. — At  most  places  in 
the  northern  hemisphere,  the  change  of  temperature  for  the  dif- 
ferent months  follows  a  law  similar  to  that  above  described  for 
New  Haven.     We  find  at  most  places  that  the  average  heat  goes 
on  increasing  from  day  to  day,  uninterruptedly  from  March  until 
some  time  in  summer,  and  after  that  time  the  mean  heat  of  each 
day  decreases  uninterruptedly  until  some  time  in  winter.     The 
time,  however,  of  the  annual  maximum  and  minimum  varies  with 
the  latitude  of  the  observer.    Near  the  equator,  the  entire  annual 
variation  of  temperature  is  very  small,  and  the  greatest  cold  may 
occur  in  any  month  from  November  to  March,  or  even  from  July 
to  September.    Indeed,  at  some  places  near  the  equator  there  are 
two  annual  maxima  of  temperature  and  two  annual  minima. 
But  in  the  extreme  southern  part  of  the  United  States,  the  great- 
est cold  usually  occurs  in  December ;  near  the  parallel  of  40°,  it 
occurs  about  the  middle  of  January  ;  in  the  northern  part  of  the 
United  States,  about  the  first  of  February  ;  at  Toronto  it  occurs 
as  late  as  the  middle  of  February ;  and  in  latitude  78°,  the  great- 
est cold  occurs  in  March. 

Throughout  most  of  the  United  States,  the  maximum  tempera- 
ture occurs  about  the  middle  of  July ;  but  at  some  places  north 
of  the  United  States,  the  maximum  does  not  occur  until  the  10th 
of  August.  See  Table  XVII. 

• 

49.  Cause  of  these  Peculiarities. — If  the  temperature  at  any  place 
depended  simply  upon  the  direct  momentary  influence  of  the  sun, 
the  maximum  would  coincide  with  the  summer  solstice;  but  dur- 
ing the  most  of  summer  the  heat  received  from  the  sun  during 
the  day  is  greater  than  the  loss  of  radiation  during  the  night,  and 
the  maximum  occurs  when  the  loss  by  night  is  just  equal  to  the 
gain  by  day.     During  the  autumn,  the  loss  by  night  is  much 
greater  than  the  gain  by  day,  and  the  mean  temperature  rapidly 
falls.     The  minimum  occurs  when  the  gain  by  day  is  just  equal 
to  the  loss  by  night,  and  this  generally  takes  place  some  time 
after  the  winter  solstice. 


TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH.     33 

The  time  of  maximum  or  minimum  temperature  depends  not 
simply  upon  the  sun's  altitude  at  noon,  but  also  upon  the  num- 
ber of  hours  during  which  the  sun  is  above  the  horizon  ;  that  is, 
upon  the  relative  length  of  the  days  and  nights.  The  minimum 
occurs  later  in  high  than  in  low  latitudes  on  account  of  the  short- 
ness of  the  winter  days  in  high  latitudes;  and  the  maximum  oc- 
curs later  on  account  of  the  greater  length  of  the  summer  days 
in  high  latitudes. 

50.  Mean  Temperature  of  a  Place. — The  mean  temperature  of  a 
year  is  found  by  taking  the  average  of  all  the  monthly  tempera- 
tures for  the  year.     This  annual  mean  is  not  the  same  every  year 
at  the  same  place;  nevertheless,  the  difference  between  the  cold- 
est and  hottest  years  seldom  exceeds  ten  degrees.    At  New  Haven 
the  hottest  year  which  has  occurred  in  a  period  of  86  years  was 
that  of  1828,  and  the  coldest  year  was  that  of  1836,  the  extreme 
range  of  the  annual  temperature  in  86  years  being  6°.3. 

At  Breslau,  in  Prussia,  the  extreme  range  of  the  annual  tem- 
perature in  66  years  has  been  ten  degrees. 

By  taking  the  average  of  the  mean  annual  temperatures  for  a 
great  number  of  years,  we  obtain  the  mean  temperature  of  a  place. 
To  determine  this  mean  temperature  with  considerable  accuracy 
for  a  variable  climate,  observations  should  be  continued  at  least 
a  quarter  of  a  century,  in  order  that  the  accidental  differences  be- 
tween successive  years  may  compensate  each  other. 

This  mean  temperature  of  a  place  is  sensibly  constant  from  one 
century  to  another,  and  there  is  no  sufficient  reason  for  believing 
that  the  mean  temperature  of  any  place  on  the  earth's  surface  has 
changed  appreciably  in  two  thousand  years. 

51.  Non-periodic  Variations. — Besides  the  periodic  variations  of 
temperature,  there  are  accidental  variations  due  to  causes  which 
will  be  mentioned  hereafter.     These  fluctuations  of  temperature 
are  frequently  experienced  simultaneously  over  large  portions  of 
the  globe ;  and  we  frequently  find  that  at  the  same  time  in  other 
parts  of  the  world  changes  of  temperature  are  observed  in  the  op- 
posite direction. 

C 


34  METEOROLOGY. 


DISTRIBUTION   OF   HEAT  OVER  THE   EARTH'S   SURFACE. 

52.  Temperature  of  different  Latitudes. — If  we  follow  a  meridian 
from  the  equator  toward  either  pole,  we  shall  find  that  the  mean 
temperature  generally  decreases,  but  not  uniformly.    On  the  con- 
trary, there  are  places  where,  as  we  proceed  toward  the  pole,  the 
mean  temperature  rises  instead  of  falling.     These  irregularities 
are  due  to  local  causes,  which  vary  upon  different  meridians,  so 
that  the  points  of  equal  mean  temperature  are  not  situated  upon 
a  parallel  of  latitude. 

53.  Isothermal  Lines. — In  order  to  represent  all  the  observa- 
tions of  temperature  conveniently  upon  a  map,  we  draw  a  line 
connecting  all  those  places  whose  mean  temperature  is  the  same. 
Such  a  line  is  called  an  isothermal  line.     In  the  neighborhood  of 
the  equator,  the  mean  annual  temperature  is  usually  about  80°. 
In  Africa  and  the  Indian  Archipelago,  the  mean  temperature  near 
the  equator  is  about  82°,  and  in  a  few  localities  it  is  still  higher. 
In  a  few  places  the  mean  temperature  of  a  single  year  has  been 
known  to  rise  to  85°,  and  even  higher.    The  area  having  a  mean 
temperature  of  80°  and  upward,  forms  a  belt  of  over  1000  miles 
in  breadth  for  more  than  half  the  circumference  of  the  globe ;  for 
about  a  quarter  of  the  circumference  this  belt  has  a  breadth  va- 
rying from  1000  miles  to  zero ;  and  for  thirty  or  forty  degrees 
of  longitude,    the  mean  temperature  near  the  equator  does  not 
exceed  79°.     See  Table  XVIII. 

The  isothermal  line  of  10°  is  a  line  gently  undulating,  but  gen- 
erally is  nearly  parallel  to  the  equator.  In  the  northern  hemi- 
sphere, this  line  passes  through  Galveston,  New  Orleans,  Mobile, 
and  St.  Augustine ;  through  the  Island  of  Teneriffe ;  through 
Alexandria,  in  Egypt;  and  Canton,  in  China. 

The  isothermal  line  of  60°  passes  through  Sacramento,  Cali- 
fornia; Memphis,  Tennessee ;  Chapel  Hill,  North  Carolina;  Nor- 
folk, Virginia;  through  the  northern  part  of  Spain  ;  Rome,  in 
Italy  ;  a  little  south  of  Constantinople ;  near  the  south  end  of  the 
Caspian  Sea ;  and  through  Shanghai,  in  China. 

The  isothermal  line  of  50°  passes  through  Puget's  Sound,  on  the 
Oregon  coast ;  through  Burlington,  Iowa ;  Pittsburg,  Pennsyl- 
vania ;  New  Haven,  Connecticut;  Dublin,  in  Ireland;  Brussels, 


TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH. 


35 


in  Belgium  ;  and  Vienna,  in  Austria;  near  the  northern  shore  of 
the  Caspian  Sea;  and  a  little  north  of  Pekin,  in  China. 

The  isothermal  line  q/"40°  passes  through  the  middle  of  Lake 


Superior;  through  Quebec;  through  Halifax,  Nova  Scotia;  a 
little  south  of  Iceland;  through  Upsala,  in  Sweden;  through  Pe- 
tersburg and  Moscow,  in  Russia. 


36 


METEOROLOGY. 


The  isothermal  line  of  32°  is  an  undulating  oval  curve,  whose 
centre  is  near  the  north  pole,  and  which  is  elongated  in  the  di- 
rection of  the  continents  of  America  and  Asia.  Over  the  conti- 
nents this  line  descends 

120°  150°       iso°          150°  to  latitude  52°,  but  on 

the  coast  of  Norway  it 
rises  as  high  as  latitude 
72°.  The  longer  diame- 
ter of  this  curve  is  near- 
ly twice  that  of  its  short- 
er. This  line  passes  a 
little  south  of  Behring's 
Straits  ;  near  the  north- 
ern shore  of  Lake  Supe- 
rior ;  through  the  south  margin  of  James's  Bay  ;  the  southern 
part  of  Greenland;  a  little  north  of  Iceland;  through  North  Cape, 
in  Norway ;  and  through  Barnaul,  in  Siberia.  Throughout  the 
entire  area  inclosed  by  this  line  the  mean  annual  temperature  is 
below  that  of  melting  ice.  All  these  lines  are  represented  on 
figures  15  and  16. 

It  is  not  claimed  that  all  of  these  lines  have  been  traced  by 
actual  observation,  and  the  positions  assigned  them  are  liable  to 
some  degree  of  uncertainty ;  but  the  observations  are  so  numer- 
ous and  so  well  distributed  as  to  leave  little  doubt  respecting  the 
approximate  position  of  the  isothermal  lines. 

54.  Mean  Temperature  of  the  North  Pole. — At  several  places  in 
the  Arctic  regions  the  mean  temperature  has  been  found  to  be 
but  little  above  zero  ;  and  at  Van  Eensselaer  Harbor,  in  latitude 
78°,  the  mean  temperature  is  two  and  a  half  degrees  below  zero. 
It  is  probable  that  near  the  north  pole  there  is  a  considerable 
area  whose  temperature  is  below  zero  of  Fahrenheit.  From  the 
form  of  the  neighboring  isothermal  lines  we  conclude  that  this 
area  is  an  oval  nearly  2000  miles  in  length  and  700  miles  in 
breadth,  and  it  lies  chiefly  on  the  American  side  of  the  north 
pole.  It  is  even  doubtful  whether  the  north  pole  is  at  all  in- 
cluded in  this  area.  The  coldest  spot  in  the  northern  hemisphere 
appears  to  be  north  of  the  American  continent  in  latitude  80°  to 
85°,  and  its  mean  temperature  is  probably  at  least  five  degrees 
below  zero.  See  Table  XIX. 


TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH.     37 

55.  Two  Sides  of  the  Atlantic  compared. — The  mean  temperature 
of  the  eastern  side  of  the  Atlantic  is  much  warmer  than  that  of 
the  western,  upon  the  same  parallel  of  latitude.     The  mean  tem- 
perature of  New  York  is  about  the  same  as  that  of  Dublin,  al- 
though Dublin  is  13°  north  of  New  York.    Near  Lake  Superior,  in 
latitude  50°,  we  find  the  same  mean  temperature  as  at  the  North 
Cape,  in  latitude  72°. 

This  high  temperature  of  the  European  coast  is  due  to  the  high 
temperature  of  the  North  Atlantic,  combined  with  the  prevalent 
westerly  winds.  By  means  of  the  Gulf  Stream,  the  waters  of  the 
equatorial  regions  are  conveyed  into  the  North  Atlantic,  and  a 
portion  of  this  warm  current  extends  northward  between  Iceland 
and  the  British  Islands,  and  continues  to  the  Arctic  Ocean.  The 
temperature  of  the  North  Atlantic  is  thus  raised  much  above 
what  is  due  to  its  latitude ;  and  since  throughout  the  middle  lati- 
tudes the  prevalent  winds  are  from  the  west,  this  heat  of  the 
ocean  is  communicated  to  places  on  the  eastern  side  of  the  At- 
lantic, but  not  to  those  on  the  western  side. 

56.  Two  Sides  of  the  Pacific  Ocean. — The  currents  of  the  Pacific 
Ocean  produce  an  effect  similar  to  the  currents  of  the  Atlantic, 
and  there  is  a  corresponding  difference  between  the  temperatures 
of  places  on  opposite  sides  of  the  Pacific  Ocean,  and  consequently 
a  marked  difference  between  the  temperatures  of  places  on  the 
Atlantic  and  Pacific  coast  of  North  America,  although  situated 
on  the  same  parallel  of  latitude.     The  isothermal  line  of  50°  is 
found  ten  degrees  of  latitude  farther  north  on  the  Pacific  coast 
than  it  is  on  the  Atlantic  coast.     Sitka,  in  latitude  57°  3',  has 
about  the  same  mean  temperature  as  Eastport,  Maine,  in  latitude 
44°  54'. 

57.  Northern  and  Southern  Hemispheres  compared. — The  mean 
temperature  of  the  northern  hemisphere  is  sensibly  higher  than 
that  of  the  southern,  from  the  equator  to  the  parallel  of  40°,  the 
difference  being  nearly  four  degrees  on  the  parallel  of  20°.     Be- 
yond the  parallel  of  40°,  the  mean  temperature  of  the  southern 
hemisphere  is  the  highest,  the  difference  being  nearly  five  degrees 
on  the  parallel  of  60°.     This  difference  in  the  temperature  of  the 
two  hemispheres  results,  at  least  in  part,  from  the  unequal  dis- 
tribution of  land  and  water  between  the  two  hemispheres,  and 


373G86 


38  METEOROLOGY. 

from  the  effect  of  ocean  currents.  In  the  North  Atlantic  Ocean 
the  gulf  stream  carries  the  water  of  the  equatorial  regions  into 
the  higher  latitudes,  and  by  counter  currents  the  water  of  the 
higher  latitudes  is  carried  back  to  the  equatorial  regions.  A 
somewhat  similar  system  of  currents  prevails  in  the  South  Atlan- 
tic Ocean.  In  consequence  of  these  currents,  the  mean  annual 
temperature  of  the  Atlantic  Ocean  is  below  that  of  the  continents 
in  the  lower  latitudes,  and  above  that  of  the  continents  in  the 
higher  latitudes.  The  currents  of  the  Pacific  Ocean  produce  a 
similar  effect.  Now  the  southern  hemisphere  contains  much  more 
water  than  the  northern.  Hence  the  cooling  effect  of  the  ocean 
currents  upon  the  equatorial  regions,  operates  upon  the  greatest 
extent  of  surface  in  the  southern  hemisphere,  and  the  mean  tem- 
perature of  this  part  of  the  southern  hemisphere  is  lower  than 
that  of  the  corresponding  part  of  the  northern  hemisphere.  The 
contrary  effect  takes  place  in  the  high  latitudes,  and  here  the 
mean  temperature  of  the  southern  hemisphere  is  higher  than  that 
of  the  corresponding  part  of  the  northern  hemisphere. 

58.  Hottest  and  Coldest  Months  compared. — The  climate  of  a  coun- 
try is  very  imperfectly  indicated  by  its  mean  temperature.  Two 
places  may  have  the  same  mean  temperature,  yet  differ  greatly  in 
their  extreme  temperatures,  and  consequently  also  in  their  vege- 
table productions.  Thus  the  mean  temperature  of  New  York  is 
the  same  as  that  of  Liverpool ;  yet  the  difference  between  the  mean 
temperature  of  the  three  summer  months  and  that  of  the  three 
winter  months  is  twice  as  great  in  New  York  as  it  is  in  Liverpool. 
Throughout  England  the  heat  of  summer  is  insufficient  to  ripen 
Indian  corn  ;  while  the  ivy,  which  grows  luxuriantly  in  England, 
can  scarcely  survive  the  severe  winters  of  New  York. 

There  are  some  places  where  the  mean  temperature  of  the 
hottest  month  of  the  year  differs  less  than  five  degrees  from  that 
of  the  coldest  month.  This  is  true  of  some  of  the  West  India 
Islands,  and  also  in  the  Indian  Archipelago.  At  Singapore,  the 
mean  temperature  of  January  differs  but  S-^0  from  that  of  July. 

On  the  contrary,  there  are  some  places  where  the  mean  tem- 
perature of  the  hottest  month  differs  50°,  80°,  and  even  100°  from 
that  of  the  coldest  month.  At  Quebec,  this  difference  amounts  to 
60° ;  at  Fort  Churchill,  on  Hudson's  Bay,  the  difference  is  86° ; 
and  at  some  places  in  Siberia  the  mean  temperature  of  January 
is  more  than  100°  below  that  of  July.  See  Tables  XX.  and  XXI 


TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH.     39 

v 

59.  Climates  either  Marine  or  Continental. — The  most  uniform 
temperature  is  found  to  prevail  upon  islands,  while  the  greatest 
range    of  temperature   prevails   in   the  interior    of  continents. 
Hence  climates  may  be  characterized  as  either  marine  or  conti- 
nental.    The  temperature  of  the  ocean  varies  but  little  from  sum- 
mer to  winter,  while  that  of  the  land  may  vary  more  than  100°. 
Hence  those  places  whose  temperature  is  mainly  controlled  by 
the  ocean  have  an  equable  climate,  while  those  which  are  but  lit 
tie  affected  by  the  ocean  have  an  extreme  climate. 

The  annual  range  of  temperature  is  much  less  on  the  eastern 
than  on  the  western  side  of  the  Atlantic,  because  the  prevalent 
winds  are  from  the  west.  Hence,  on  the  western  coast  of  the  At- 
lantic, where  the  prevalent  winds  come  from  the  land,  the  climate 
is  essentially  continental,  but  upon  the  eastern  side  of  the  Atlan- 
tic, where  the  prevalent  winds  come  from  the  sea,  the  climate  is 
mainly  controlled  by  the  ocean. 

60.  Highest,  observed  Temperature. — Although  the  highest  mean 
temperature  is  found  near  the  equator,  yet  the  thermometer  fre- 
quently rises  higher  in  the  middle  latitudes  than  it  does  at  many 
places  under  the  equator.    Thus,  at  Singapore,  under  the  equator, 
the  thermometer  never  rises  above  95°,  while  at  New  York  and 
at  Paris  the  thermometer  has  been  known  to  rise  to  104°.     At 
Mosul,  in  Armenia,  the  thermometer  has  been  known  to  rise  to 
117°  ;  at  Fort  Miller,  California,  to  121° ;  in  India,  to  132° ;  and 
on  the  Great  Desert  of  Africa,  to  133°. 

These  numbers  are  supposed  to  indicate  the  temperature  of  the 
air  where  it  circulates  most  freely.  A  thermometer  exposed  to 
the  direct  rays  of  the  sun  often  rises  much  higher  than  the  pre- 
ceding numbers.  In  India,  a  thermometer  whose  bulb  was  cov- 
ered with  black  wool  rose  in  the  sun  to  164° ;  and  a  thermome- 
ter placed  inside  of  a  blackened  box,  covered  with  glass,  has  been 
known  to  rise  to  248°. 

61.  Lowest  observed  Temperature. — The  lowest  temperatures  any 
where  observed  have  been  in  North  America  and  Siberia.     The 
lowest  temperature  observed  at  Singapore  is  66° ;  at  Key  "West, 
45°;  at  Paris  and  New  York,— 10°;  at  New  Haven, —24°;  and  at 
Montreal,  —38°.    At  New  Lebanon,  New  York,  atFranconia,  New 
Hampshire,  and  at  several  places  in  New  England,  mercury  froze 


40 


METEOROLOGY. 


in  January,  1835,  indicating  a  temperature  of  40°  below  zero. 
Dr.  Kane,  in  latitude  78°,  observed  a  temperature  of  67°  below 
zero ;  and  Captain  Back,  at  Fort  Eeliance,  in  latitude  62°,  ob- 
served a  temperature  of  70°  below  zero ;  while  in  Siberia  the 
thermometer  has  been  known  to  fall  to  76°  below  zero. 

62.  Range  of  Temperature. — By  combining  these  results,  we  find 
that  at  Singapore  the  entire  range  of  the  thermometer  is  only 
29°,  while  at  New  York  it  is  114° ;  at  Montreal  it  is  140°,  and 
at  Fort  Reliance,  in  latitude  62°,  the  thermometer  in  four  months 
varied  from  —70°  to  +81,  being  a  range  of  151°. 

The  entire  range  of  the  temperature  of  the  air  any  where  ob- 
served is  from  -76  to  +133°,  or  209°. 

The  range  of  temperature  for  a  single  day  in  the  middle  lati- 
tudes is  often  greater  than  for  a  whole  year  in  the  equatorial  re- 
gions. At  Hanover,  New  Hampshire,  February  7, 1861,  at  noon, 
the  thermometer  stood  at  40°;  the  next  morning  it  stood  at  —32°, 
making  a  range  of  72°  in  18  hours.  See  Tables  XXII.  and  XXIII. 

TEMPERATURE   OF  THE  AIR  AT  DIFFERENT  HEIGHTS. 

63.  Change  of  Temperature  with  Elevation. — As  we  ascend  above 
the  surface  of  the  earth  the  mean  temperature  of  the  air  declines. 
This  depression  of  temperature  is  observed  when  we  ascend  a 
mountain  or  rise  in  a  balloon.     The  rate  of  decrease  varies  with 
the  latitude  of  the  place,  with  the  season  of  the  year,  as  well  as 
the  hour  of  the  day.     It  is  more  rapid  in  warm  countries  than  in 
cold  countries,  and  is  most  rapid  during  the  hottest  months.     It 
is  most  rapid  about  5  P.M.,  and  least  rapid  about  sunrise. 

The  change  is  also  most  rapid  near  the  earth's  surface,  and  di- 
minishes as  we  ascend.  From  a  long  series  of  balloon  ascents, 
made  under  the  direction  of  the  British  Scientific  Association,  the 
following  results  have  been  obtained  for  the  vicinity  of  London : 


Elevation. 

When  the  Sky  is 
clear. 

When  the  Sky  is 
cloudy. 

Fr 

Dm         0  ft. 
5000 
10000 
15000 
20000 
25000 

to    5000  ft.,  tl 
10000 
15000 
20000 
25000 
30000 

ie  decrea 

se  is  1 

for  239  ft.  elf 
394 
490 
581 
877 
1190 

;v'n. 

1°  for  271  ft.  elev'n. 
"    394         " 
"    459         " 
"    725         " 
"  1111         " 

TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH. 


64.  Cause  of  this  Decrease  of  Temperature. — This  decrease  of  tem- 
perature as  we  rise  above  the  earth's  surface  is  mainly  due  to  the 
expansion  of  the  air.     The  lower  strata  of  the  air,  being  heated 
by  the  sun  (Art.  38)  and  expanded,  tend  to  rise  in  consequence 
of  their  diminished  specific  gravity.    As  the  air  ascends  it  is  sub- 
ject to  a  diminished  pressure  and  expands;  its  heat  is  diffused 
through  a  greater  amount  of  space,  by  which  means  a  part  of  its 
sensible  heat  becomes  latent. 

This  principle  may  be  proved  experimentally  by  placing  a 
thermometer  under  the  receiver  of  an  air-pump  and  rapidly  ex- 
hausting the  air,  when  the  thermometer  indicates  a  diminution  of 
sensible  temperature.  Upon  readmitting  the  air  the  thermometer 
rises  to  its  former  height. 

The  atmosphere  would  be  in  a  condition  of  equilibrium  if  a 
pound  of  air  at  all  elevations,  whether  on  the  summit  of  a  mount- 
ain or  at  the  level  of  the  sea,  contained  the  same  amount  of  heat. 
The  atmosphere  is  perpetually  seeking  to  attain  to  this  condition 
of  equilibrium,  but  since  the  sun  perpetually  acts  as  a  disturbing 
force,  such  an  equilibrium  is  never  fully  attained. 

65.  Law  of  decrease  of  Temperature  ivith  Height. — We  see  from 
the  observations  of  Art.  63  that  the  diminution  of  temperature  is 
not  proportional  to  the  height ;  but  we  find  that  the  temperature 
is  intimately  connected  with  the  pressure,  as  shown  by  the  ba- 
rometer.    The  following  table  presents  a  summary  of  these  ob- 
servations for  a  clear  sky : 


Barometer. 

Temperature. 

Difference. 

Barometer. 

Temperature. 

Difference. 

10  incheg. 
12 

-10°.9 
-6   .1 

4°.8 

20  inches. 
22 

15°.  3 
21   .0 

5°.  7 

14 

-1    .7 

4  .4 

24 

26  .8 

5  .8 
6    a 

16 

+3  .7 

r     a 

26 

32  .7 

7     2 

18 

+9  .5 

K     a 

28 

39  .9 

10     1 

20 

+  15  .3 

30 

50  .0 

Column  first  shows  the  pressure  indicated  by  the  barometer, 
and  column  second  the  corresponding  temperature  when  the  tem- 
perature at  the  earth's  surface  was  50°.  The  third  column  shows 
the  change  of  temperature  corresponding  to  a  change  of  two 
inches  in  the  pressure.  These  differences  are  greatest  near  the 
earth's  surface,  but  after  rising  one  mile  they  become  nearly  con- 
stant ;  that  is,  the  fall  of  the  thermomete3'  is  nearly  proportional  to 
the  fall  of  the  barometer,  the  change  of  the  thermometer  being 


42 


METEOROLOGY. 


about  five  degrees  for  a  change  of  pressure  amounting  to  two 

inches. 

Fig.  IT.  The  curve  in  Fig.  17  shows  more 

12  readily  how  the  temperature  de- 
g  pends  upon  the  pressure.  The  ab- 
is  scissas  represent  the  observed  tern- 
22  peratures  from  50°  to  —11°,  and  the 
^*  ordi nates  show  the  corresponding 
28  pressures  from  thirty  inches  to  ten, 
inches. 


80°       40°      30°      20°  +10D       0    —10 


66.  Limit  of  Perpetual  Snow. — In  consequence  of  this  decrease 
of  temperature,  the  summits  of  high  mountains,  even  within  the 
tropics,  are  always  covered  with  snow.  The  limit  of  perpetual 
snow  is  not  the  line  whose  mean  temperature  is  32°.  The  snow- 
line  is  determined  more  by  the  mean  temperature  of  the  hottest 
month  than  by  the  mean  temperature  of  the  year. 

The  limit  of  perpetual  snow  generally  descends  as  we  proceed 
from  the  equator  toward  the  poles,  but  there  are  many  exceptions 
to  this  rule.  The  height  of  the  snow-line  depends  upon  a  variety 
of  circumstances :  not  only  upon  the  mean  temperature,  but  upon 
the  extreme  heat  of  summer ;  upon  the  amount  of  the  annual  fall 
of  snow;  upon  the  prevalent  winds;  and  upon  the  proximity  of 
mountain  peaks  or  extensive  plains.  Under  the  equator  the 
height  of  the  snow-line  varies  from  15,000  to  16,000  feet,  where 
the  mean  annual  temperature  is  35°.  On  the  Alps,  the  average 
height  of  the  snow-line  is  8800  feet,  where  the  mean  annual  tem- 
perature is  25° ;  while  on  the  coast  of  Norway  its  height  is  only 
2400  feet,  where  the  mean  annual  temperature  is  21°.  Fig.  18 
shows  the  snow-line  on  several  mountains  in  different  latitudes. 

Fig.  18. 


Numbers  1,  2,  and  3  are  the  Illimani,  Aconcagua,  and  Chimbo 


TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH.     43 

razo,  in  South  America ;  4,  5,  and  6  are  the  Choomalari,  Dhaula- 
giri,  and  Caucasus,  in  Asia ;  number  7  is  the  Pyrenees,  and  8  the 
Alps ;  number  9  the  Sulitelma,  in  Norway ;  and  number  10  the 
island  Mageroe.  See  Table  XXIY. 

67.  Temperature  of  the  Interplanetary  Spaces. — The  temperature 
of  the  air  does  not  continue  to  sink  indefinitely  as  we  rise  above 
the  earth's  surface.     Its  mean  temperature  can  nowhere  fall  be- 
low the  temperature  of  the  interplanetary  spaces.     The  space  in 
which  the  planets  move  has  a  temperature  of  its  own,  due  to  the 
radiation  of  heat  from  the  stars,  each  of  which  is  a  hot  body  like 
our  sun.    This  temperature  of  space  is  necessarily  lower  than  the 
mean  temperature  of  the  polar  regions  of  the  earth,  for  during  six 
months  of  the  year  these  are  illumined  by  the  sun,  from  which 
they  derive  a  large  amount  of  heat. 

68.  Mode  of  estimating  its  Amount. — The  temperature  of  celes- 
tial space  must  be  lower  than  that  of  the  polar  regions  during  the 
coldest  months  of  the  year,  for  during  winter  these  regions  do 
not  lose  all  the  heat  received  from  the  sun  during  the  preceding 
summer,  and  by  means  of  winds  there  is  a  constant  interchange 
of  heat  between  the  polar  and  equatorial  regions  of  the  earth. 

Now  at  Jakutsk,  in  Siberia,  the  mean  temperature  of  the  month 
of  January  is  44°  below  zero.  Moreover,  from  October  to  No- 
vember, the  temperature  of  that  place  sinks  34° ;  from  Novem- 
ber to  December  it  sinks  18°,  and  from  December  to  January  6°. 
If  the  sun's  heat  were  to  be  permanently  withdrawn,  the  temper- 
ature would  doubtless  fall  still  lower  than  is  now  observed  in 
January,  probably  as  low  as  —60°.  We  can  not  then  suppose 
the  temperature  of  space  to  be  higher  than  —60°. 

Many  different  methods  have  been  employed  for  estimating  the 
temperature  of  space.  The  average  of  the  estimates  of  several 
distinguished  philosophers  makes  it  as  low  as  —  80°. 

69.  The  Atmosphere  a  regulator  of  Tem.pera.iure. — The  atmos- 
phere serves  as  a  regulator  of  the  sun's  heat.     During  the  day  it 
absorbs  a  portion  of  the  sun's  rays,  by  which  it  is  warmed,  and  as 
it  expands  a  part  of  the  heat  becomes  latent.     During  the  night 
the  air  intercepts  a  part  of  the  rays  emitted  by  the  earth,  and  as 
it  cools  it  contracts,  and  restores  to  the  sensible  condition  the  la- 


44  METEOROLOGY. 

tent  heat  which  it  had  absorbed  during  the  day.  Without  an 
atmosphere  we  should  experience  during  the  day  an  excessive 
heat  from  the  sun's  rays,  no  portion  of  which  would  be  inter- 
cepted, and  during  the  night  an  intense  cold  resulting  from  the 
unobstructed  radiation  of  heat  into  space. 

TEMPERATURE   OF  THE   EARTH   AT   DIFFERENT   DEPTHS. 

70.  Means  of  Observation. — For  the  purpose  of  measuring  the 
variations  of  temperature  beneath  the  surface  of  the  earth,  ther- 
mometers with  very  long  stems  have  been  buried  at  different 
depths  in  the  ground,  the  stem  being  of  such  a  length  as  to  rise 
above  the  surface  of  the  earth,  so  that  the  temperature  can  be 
observed  without  disturbing  the   position  of  the  thermometer. 
For  convenience  of  comparison,  it  has  generally  been  agreed  to 
adopt  a  uniform  system,  and  thermometers  have  been  buried  at 
depths  of  24, 12,  6,  and  3  French  feet.     [A  French  foot  is  about 
-jijth  greater  than  an  English  foot.]     From  twenty  to  thirty  years 
ago,  thermometers  were  buried  at  these  four  depths  at  Brussels, 
Edinburg,  Greenwich,  and  other  places  ;  and  other  thermometers 
were  also  buried  at  depths  less  than  three  feet.     At  first,  these 
thermometers  were  observed  several  times  each  day,  but  after- 
ward once  a  day  or  once  a  week. 

71.  Range  of  the  fluctuations  of  Temperature. — Since  the  earth  is 
a  bad  conductor  of  heat,  the  range  of  the  fluctuations  of  temper- 
ature rapidly  diminishes  as  we  descend  below  the  surface.     At 
a  certain  depth  the  diurnal  variations  of  temperature  disappear, 
and  at  a  greater  depth  the  annual  variations   also  disappear. 
These  depths  vary  as  the  square  root  of  the  period  compared. 
The  annual  variations  disappear  at  a  depth  19  times  greater  than 
the  diurnal  variations ;  19  being  nearly  the  square  root  of  365, 
the  number  of  days  in  a  year.     In  Europe  generally,  the  diurnal 
variations  are  not  sensible  to  a  greater  depth  than  8-3-  feet ;  but 
the  depth  varies  somewhat  with  the  latitude  and  the  conducting 
power  of  the  soil. 

At  the  depth  of  three  feet  the  annual  range  of  temperature  is 
less  than  half  what  it  is  at  the  surface ;  at  the  depth  of  twelve 
feet  it  is  less  than  one  fourth,  and  at  the  depth  of  twenty-four 
feet  it  is  less  than  one  tenth  what  it  is  at  the  surface. 


TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH.     45 

72.  Stratum  of  Invariable  Temperature. — At  a  certain  depth  the 
annual  variations  of  temperature  become  insensible  ;   that  is,  we 
find  a  temperature  which  is  invariable  from  summer  to  winter. 
This  depth  depends  upon  the  extreme  range  of  the  temperature 
of  the  air.     In  Europe  it  is  from  80  to  100  feet  beneath  the  sur- 
face.    A  thermometer  which  has  been  kept  for  75  years  in  the 
vaults  of  the  Observatory  at  Paris,  at  the  depth  of  91  feet  below 
the  surface,  has  not  varied  more  than  half  a  degree  during  the 
entire  interval. 

The  annual  mean  of  the  temperatures  observed  at  different 
depths  is  very  nearly  the  same  as  that  of  the  air.  Hence  we  are 
furnished  with  a  convenient  means  of  determining  nearly  the 
mean  temperature  of  any  locality,  and  this  method  is  one  of  great 
value  to  scientific  travelers. 

73.  Time  of  Maximum  and  Minimum  Temperature. — Since  the 
earth  is  a  bad  conductor  of  heat,  the  heat  of  the  sun  penetrates 
the  ground  slowly,  and  the  highest  temperature  of  the  year  oc- 
curs later  and  later  the  deeper  we  descend  below  the  surface.    At 
the  depth  of  twelve  feet  the  maximum  temperature  of  the  year 
does  not  occur  until  October,  and  the  minimum  occurs  in  April. 
At  the  depth  of  twenty-four  feet,  the  maximum  occurs  in  Decem- 
ber, and  the  minimum  in  June  or  July.     These  dates  vary  some- 
what in  different  countries,  being  dependent  upon  the  conduct- 
ing power  of  the  soil. 

The  maximum  of  daily  temperature  also  occurs  later  the  deep- 
er we  descend,  requiring  three  hours  to  penetrate  to  a  depth  of 
four  inches. 

74.  Increase  of  Temperature  with  the  Depth. — Below  the  depth 
of  100  feet  from  the  surface,  we  find  an  invariable  temperature 
throughout  the  year ;  but  this  temperature  is  not  the  same  as  the 
mean  temperature  at  the  surface.     Numerous  observations  have 
been  made  in  different  parts  of  the  globe,  and  they  invariably 
indicate  that  the  mean  temperature  increases  with  the   depth. 
These  observations  have  been  extended  to  very  great  depths  by 
means  of  mines  and  artesian  wells.     An  artesian  well  consists 
of  a  shaft  of  a  few  inches  in  diameter,  bored  into  the  earth  till  a 
spring  is  found.     To  prevent  the  water  from  being  carried  off  by 
the  adjacent  strata,  a  tube  is  generally  inserted,  exactly  fitting  the 


I 


46  METEOROLOGY. 

bore  from  top  to  bottom,  and  through  this  tube  the  water  rises  to 
the  surface.  Artesian  borings  have  been  made  in  Europe  to  a 
depth  of  more  than  2300  feet  below  the  surface,  and  some  of  the 
mines  are  more  than  2000  feet  deep. 

In  Europe  the  average  increase  of  temperature  deduced  from 
mines  and  artesian  wells  is  one  degree  for  a  descent  of  52  feet. 

75.  Rate  of  Increase  in  the  United  States. — Some  very  deep  bor- 
ings have  been  made  in  the  United  States.     An  artesian  well  in 
Charleston, South  Carolina,  has  a  depth  of  1000  feet;  one  in  Lou- 
isville, Kentucky,  has  a  depth  of  2086  feet ;  a  third  in  St.  Louis, 
has  a  depth  of  2200  feet;  and  a  fourth  in  Columbus,  Ohio,  has  a 
depth  of  2575  feet.     The  boring  at  Louisville  indicates  an  in- 
crease of  temperature  of  one  degree  for  every  76  feet ;  and  that 
at  Columbus  gives  an  increase  of  one  degree  for  every  71  feet. 
The  mean  of  these  two  experiments  gives  an  increase  of  one  de- 
gree for  every  73  feet,  which  is  less  than  the  rate  of  increase  in 
Europe. 

76.  Stratum  of  Frozen  Earth. — Throughout  nearly  the  whole  of 
the  Arctic  circle  the  mean  temperature  is  considerably  below  32°, 
and  this  is  also  the  mean  temperature  of  the  surface  of  the  earth. 
Now,  in  the  polar  regions,  the  earth  in  summer  only  thaws  to  a 
depth  of  three  or  four  feet.     Below  this  line  is  a  stratum  of  per- 
manent frost,  whose  depth  increases  as  we  advance  northward, 
the  lower  limit  being  determined  by  the  increase  of  temperature 
explained  in  Art.  74.     At  Jakutsk,  latitude  62°  2',  it  has  been  de- 
termined by  actual  excavation  that  the  earth  is  frozen  to  a  depth 
of  382  feet.     In  the  polar  regions,  therefore,  wells  are  impossible, 
unless  they  are  sunk  to  a  depth  below  that  of  the  permanent  frost. 

77.  Temperature  of  the  Earth  at  great  Depths. — If  the  tempera- 
ture of  the  earth  at  great  depths  increases  at  the  same  rate  as 
near  the  surface,  at  a  depth  of  two  miles  the  temperature  must 
exceed  that  of  boiling  water,  and  at  a  depth  of  less  than  a  hund- 
red miles  the  rocks  must  be  in  a  state  of  fusion.     We  are  thus 
led  to  the  conclusion  that,  with  the  exception  of  a  comparatively 
thin  crust  upon  the  surface,  the  entire  mass  of  the  earth  is  proba- 
bly in  a  state  of  igneous  fusion. 


TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH.     47 

78.  Information  furnished  by  Volcanoes. — This  conclusion  is  con- 
firmed by  the  phenomena  of  volcanoes.     At  numerous  points 
upon  the  earth's  surface  we  find  volcanoes  which  frequently  eject 
immense  masses  of  melted  rock,  and  which,  without  doubt,  at  all 
times  contain  large  quantities  of  rock  in  a  state  of  fusion.     Vol- 
canoes, extinct  or  active,  border  the  Pacific  Ocean  from  Cape 
Horn  to  the  Arctic  circle ;  thence  they  extend  in  a  line  to  Asia, 
and  along  the   coast  of  Japan  to  the  Philippine  Islands,  New 
Guinea,  and  New  Zealand  ;  and  they  constitute  half  of  the  isl- 
ands of  the  Pacific  Ocean.     Volcanoes  occur  also  in  Central  and 
Western  Asia ;  in  Southern,  Central,  and  Southwestern  Europe  ; 
in  Iceland  and  the  West  Indies.     Volcanoes  therefore  are  so  nu- 
merous (their  number  exceeding  500)  as  to  indicate  that  a  con- 
siderable portion  of  the  interior  of  the  earth  must  be  in  a  state 
of  fusion. 

Some  have  doubted  whether  the  whole  interior  of  the  earth  is 
in  a  state  of  fusion,  and  are  disposed  to  admit  only  the  existence 
of  interior  seas  of  liquid  rock. 

79.  Observations  of  Hot  Springs. — At  many  places  remote  from 
any  active  volcano  we  find  natural  springs  which  emit  water  of  a 
very  high  temperature.     Many  of  the  springs  of  Germany  have 
a  temperature  of  140  to  150  degrees,  and  one  has  a  temperature 
of  167°.     At  New  Lebanon,  N.  Y.,  is  a  spring  whose  temperature 
is  25°  above  the  mean  temperature  of  the  place.     A  spring  in 
Virginia  has  a  temperature  of  108°,  another  in  North  Carolina  has 
a  temperature  of  125°,  while  one  in  Arkansas  has  a  temperature 
of  148°.     Near  San  Francisco  is  a  spring  which  perpetually  emits 
boiling  water,  and  there  is  a  similar  one  near  the  eastern  bound- 
ary of  California.     These  springs  probably  rise  from  great  depths, 
and  are  proofs  of  the  increasing  temperature  of  the  earth  as  we 
descend  below  the  surface. 

80.  Temperature  of  Ordinary  Springs.  —  The  ordinary  springs 
and  wells  of  a  country  afford  a  convenient  means  of  determining 
approximately  its  mean  temperature.     The  mean  temperature  of 
the  water  proceeding  from  springs  is  nearly  that  of  the  strata  from 
which  they  rise.     Hence  the  water  from  deep  springs  preserves 
throughout  the  year  a  nearly  uniform  temperature,  and  this  is 
generally  a  little  above  the  mean  temperature  of  the  air.     This 


48  METEOROLOGY. 

difference  may  amount  to  five  or  six  degrees;  and,  on  the  contra- 
ry, the  mean  temperature  of  springs  is  sometimes  a  little  btlow 
that  of  the  air.  The  temperature  of  springs  is  modified  by  the 
temperature  of  the  rain  which  supplies  them.  In  those  places 
where  the  rain  falls  chiefly  in  summer,  the  mean  temperature  of 
springs  should  be  higher  than  that  of  the  air,  but  it  should  be 
lower  in  those  countries  where  the  rain  falls  chiefly  in  winter. 
Hence  great  caution  is  required  in  deducing  the  mean  tempera- 
ture of  a  place  from  the  temperature  of  its  springs. 

81.  Low  Temperature  of  certain  Wells. — In  some  wells  the  mean 
temperature  of  the  water  is  considerably  below  the  mean  temper- 
ature of  the  place.     In  ordinary  wells  the  water  is  in  continual 
circulation,  the  water  of  the  well  flowing  off  by  underground 
streams,  while  fresh  water  flows  in  through  similar  channels. 
Thus  throughout  the  year  the  water  of  the  well  preserves  nearly 
the  temperature  of  the  earth  at  the  same  depth ;  and  a  few  ob- 
servations of  such  a  well  will  furnish  very  nearly  the  mean  tem- 
perature of  the  place.     But  in  some  wells  there  is  very  little  cir- 
culation, the  same  water  remaining  in  the  well  for  a  long  time 
with  but  trifling  change.     Now,  since  cold  air  is  heavier  than 
warm  air,  the  cold  air  of  winter  descends  into  the  well,  and  com- 
municates its  own  temperature  to  the  water  in  the  well.     The 
water  thus  becomes  chilled,  and  it  may  even  freeze,  as  actually 
happens  to  many  wells  of  New  York  and  New  England.     When 
considerable  ice  once  forms  in  a  well,  it  must  remain  for  a  long 
time  unmelted,  because  in  summer  the  warm  external  air  can  not 
displace  the  heavier  cold  air  of  the  well.    Under  such  circumstan- 
ces, ice  has  been  known  to  continue  till  after  midsummer ;   and 
the  mean  temperature  of  such  a  well  may  be  several  degrees  be- 
low the  mean  temperature  of  the  place. 

82.  Remarkable  Examples. — In  Brandon,  Yt,  is  a  well  34  feet 
deep,  in  which,  during  the  winter,  ice  forms  six  or  eight  inches 
in  thickness,  and  does  not  entirely  disappear  until  the  close  of  the 
succeeding  summer. 

In  Owego,  N.  Y.,  was  formerly  a  well  77  feet  in  depth,  where 
ice  formed  during  the  winter,  and  has  been  known  to  continue 
until  near  the  close  of  July. 


TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH.     49 

83.  Natural  Ice-houses. — In  hilly  countries  we  sometimes  find 
secluded  spots  where  the  ice  which  accumulates  in  winter  is  so 
protected  against  the  action  of  the  sun  in  summer  that  it  remains 
unmelted  till  August,  or  perhaps  even  through  the  year.     The 
springs  which  flow  from  such  places  may  show  a  temperature  but 
little  above  32°,  even  in  midsummer.     Several  examples  of  this 
kind  are  found  in  New  England,  and  still  more  remarkable  ex- 
amples are  found  in  the  mountainous  districts  of  Europe.     On 
the  western  bank  of  Lake  Champlain,  near  the  village  of  Port 
Henry,  is  an  iron  mine  in  which  the  ice  accumulates  in  winter, 
and  does  not  entirely  disappear  during  the  subsequent  season. 
In  Meriden,  Conn.,  is  a  rocky  ledge  of  little  elevation,  where  the 
ice  of  winter  remains  unmelted  until  the  succeeding  August. 

In  the  eastern  part  of  France  (Besan9on),  at  an  elevation  of 
less  than  3000  feet  above  the  sea,  is  a  cavern  where  the  ice  has 
been  known  to  lie  unmelted  for  more  than  a  century. 

84.  Temperature  of  the  Sea.  —  To  determine  the  temperature 
of  the  sea  at  different  depths  we  require  some  kind  of  self-reg- 
istering thermometer.      The  instrument  employed  for  such  ob- 
servations in  the  U.  S.  Coast  Survey  is  Saxton's  metallic  ther- 
mometer. 

This  instrument  consists  of  a  compound  coil  or  helix  about  six 
inches  in  length,  formed  of  two  stout  ribbons  of  silver  and  plati- 
num, with  an  intermediate  thin  plate  of  gold,  all  soldered  togeth- 
er, the  silver  being  on  the  inside  of  the  coil.  One  end  of  this  coil 
is  firmly  attached  to  the  base  of  a  cylinder,  while  the  other  end 
is  fastened  to  a  brass  stem  passing  through  the  axis  of  the  coil. 
"When  the  temperature  rises,  the  curvature  of  each  spiral  dimin- 
ishes, because  the  silver  expands  more  than  the  platinum ;  and 
when  the  temperature  declines,  the  curvature  of  each  spiral  in- 
creases. The  coil  therefore  winds  and  unwinds  with  the  varia- 
tions of  temperature,  and  this  motion  gives  rotation  to  the  brass 
stem.  This  motion  is  registered  upon  the  dial  of  the  instrument 
by  an  index  which  pushes  before  it  a  registering  hand,  moving 
with  friction  barely  sufficient  to  retain  its  place  when  thrust  for- 
ward by  the  index  of  the  thermometer.  The  instrument  may 
thus  be  made  to  register  both  the  highest  and  lowest  tempera- 
tures to  which  it  has  been  exposed. 

D 


60  METEOROLOGY. 

85.  Temperature  at  the  Surface  of  the  Sea. — The  surface  of  the 
sea  becomes  heated  less  readily  than  the  earth :  1st,  because  the 
rays  of  the  sun  penetrate  the  ocean  to  a  considerable  depth,  and 
therefore  produce  less  effect  at  the  surface ;  2d,  because  water  has 
a  much  greater  capacity  for  heat  than  dry  earth  ;  and,  3d,  because, 
by  the  agitation  of  the  sea,  there  is  a  perpetual  mingling  of  the 
surface  water  with  the  lower  strata.     The  surface  also  becomes 
cooled  very  slowly  for  the  same  reasons,  and  also  because,  when 
the  particles  of  the  surface  are  cooled,  they  descend,  to  be  re- 
placed by  warmer  particles  from  beneath. 

Hence  the  diurnal  variations  of  the  temperature  of  the  sea  are 
quite  small,  amounting  to  only  two  or  three  degrees  in  the  torrid 
zone,  and  4°  or  5°  in  the  temperate  zones.  The  minimum  oc- 
curs about  sunrise,  and  the  maximum  about  noon. 

Near  the  middle  of  the  Atlantic  Ocean,  under  the  equator,  the 
mean  temperature  of  the  sea  is  80°.4.  As  we  recede  from  the 
equator  northward,  the  isotherms  incline  from  northwest  to  south- 
east ;  but  after  passing  the  parallel  of  40°  they  incline  from  south- 
west to  northeast,  and  the  isotherm  of  42°  which  passes  through 
Halifax  meets  the  meridian  of  Greenwich  near  the  Arctic  circle. 

The  entire  range  of  temperature  for  the  middle  of  the  Atlantic 
during  the  year,  near  the  equator,  is  about  10° ;  near  latitude 
30°  it  is  15° ;  near  latitude  40°  it  is  20°,  and  near  latitude  50°  it 
is  24°,  which  is  scarcely  one  half  the  annual  range  of  temperature 
of  the  most  equable  climates  in  the  same  latitude  on  land. 

86.  Temperature  at  different  Depths. — Between  the  tropics  the 
temperature  of  the  sea  decreases  as  we  descend,  at  first  rapidly, 
but  afterward  more  slowly,  to  the  depth  of  over  1000  fathoms, 
where  the  thermometer  has  been  found  to  indicate  36°.     Beyond 
latitude  25°,  the  decrease  of  temperature  with  the  depth  is  less 
rapid ;  and  beyond  latitude  65°,  during  winter,  the  temperature 
sometimes  increases  as  we  descend.     When  the  temperature  of 
the  surface-water  was  28°,  the  temperature  at  the  depth  of  700 
fathoms  has  been  found  to  be  36°. 

In  very  deep  water,  all  over  the  globe,  there  is  found  to  pre- 
vail a  uniform  temperature  of  36°  to  39°.  The  depth  at  which 
this  temperature  is  found  is  about  7200  feet  at  the  equator,  and 
about  4500  feet  in  the  highest  accessible  latitudes. 


TEMPERATURE   OF  THE   AIR  AND   OF  THE   EARTH.  51 

87.  Currents  of  the  Sea. — On  the  surface  of  the  Atlantic  Ocean, 
near  the  equator,  there  is  a  current  setting  westward,  which  di- 
vides where  it  meets  the  projecting  coast  of  South  America,  one 
portion  turning  northward  and  the  other  southward.     The  for- 
mer gives  rise  to  the  Gulf  Stream,  which  travels  along  the  coast 
of  the  United  States  to  latitude  45°,  whence  a  portion  proceeds 
northeastwardly  between  Iceland  and  the  British  Islands,  and  the 
other  portion  descends  along  the  western  coast  of  Europe  and 
Africa,  and  rejoins  the  equatorial  waters. 

The  Brazil  current  coasts  along  the  South  American  shore, 
and  in  the  South  Atlantic  makes  a  circuit  somewhat  similar  to 
that  of  the  Gulf  Stream  in  the  north. 

In  the  Pacific  Ocean,  a  current  setting  westward  prevails 
throughout  the  whole  of  the  equatorial  belt  until  near  the  Asi- 
atic coast,  where,  as  in  the  Atlantic,  it  divides,  and  one  portion, 
called  the  Japan  current,  imitates  in  the  North  Pacific  the  course 
of  the  Gulf  Stream  in  the  North  Atlantic.  The  larger  portion 
of  the  equatorial  current  is,  however,  carried  southward  to  sweep 
the  northern  and  western  coast  of  Australia. 

At  the  bottom  of  the  ocean  there  prevail  counter-currents,  which 
carry  from  the  poles  toward  the  equator  the  cold  waters  of  the 
Arctic  Seas.  The  existence  of  these  currents  is  perceived  when- 
ever we  sink  a  weight  to  a  great  depth  by  means  of  a  long  cord. 
This  is  the  cause  of  the  low  temperature  prevailing  in  tropical 
regions  near  the  bottom  of  the  ocean. 

88.  Temperature  of  Banks. — Where  the  sea  is  shallow  the  water 
is  generally  found  somewhat  colder  than  in  the  adjacent  open 
ocean,  the   difference   frequently   amounting  to  ten  degrees  or 
more.     This  change  of  temperature  is  very  noticeable  over  the 
Banks  of  Newfoundland,  in  contrast  with  the  Gulf  Stream,  which 
flows   near  their  eastern  margin,  where  we  frequently  find  a 
change  of  temperature  of  33°  within  a  distance  of  300  miles. 
Thus  a  thermometer  may  frequently  give  warning  of  approach- 
ing land  in  the  darkness  of  night,  when  nothing  else  would  indi- 
cate it. 

This  low  temperature  over  banks  has  been  ascribed  to  the  un- 
der-current from  the  polar  regions  toward  the  equator,  which  in 
deep  water  is  only  found  at  great  depths,  but  which  in  shallow 
water  is  partially  forced  upward,  so  as  to  affect  somewhat  th0 
temperature  at  the  surface. 


52 


METEOROLOGY. 


89.  Polar  Ice.  —  From  latitude  40°  to  50°,  during  winter,  the 
water  of  the  ocean  freezes  somewhat  near  the  shore ;  but  it  is 
only  in  the  polar  regions  that  we  find  firm  ice  at  a  great  distance 
from  the  land.  Sea  water  freezes  at  a  temperature  of  27^°,  and 
since,  during  winter,  the  mean  temperature  of  the  polar  regions 
is  considerably  below  zero  of  Fahrenheit,  ice  forms  even  in  the 
open  sea  with  great  rapidity,  and  sometimes  attains  a  thickness 
of  twenty-five  feet.  In  the  spring  of  the  year  this  ice  is  partially 
dissolved ;  it  is  then  broken  up  by  tides  and  currents,  and  by 
northerly  winds  is  driven  into  the  open  sea,  sometimes  forming 
a  field  of  ice  100  miles  in  length  and  50  miles  in  breadth,  with  a 
thickness  of  20  or  25  feet  During  the  months  of  May  and  June 
this  ice  is  annually  encountered  in  immense  fields  off  the  coast 
of  Newfoundland,  near  the  track  of  vessels  from  New  York  to 
Liverpool. 

In  connection  with  these  immense  fields  of  comparatively  thin 
ice  are  generally  found  some  masses  of  ice  called  icebergs,  some- 
times rising  200  feet  above  the  water,  and  descending  to  a  depth 
of  1000  feet  beneath  the  surface.  These  masses  are  detached 
from  the  coasts,  around  which,  in  winter,  the  ice  accumulates  in 
cliffs  of  vast  height  and  extent.  The  largest  of  them  are  de- 
tached portions  of  vast  glaciers,  such  as  abound  on  the  precipi- 
tous coast  of  Greenland  and  Spitzbergen,  which  were  broken  off 
either  by  their  own  weight  or  the  action  of  the  waves,  and  then 
transported  by  winds  and  currents  to  the  lower  latitudes.  Fig. 


Fig.  19. 


19  represents  an  iceberg  encountered  some  years  since  near  th.9 
Cape  of  Good  Hope. 


TEMPERATURE  OF  THE  AIR  AND  OF  THE  EARTH.     53 

90.  Temperature  of  Lakes  and  Rivers. — The  temperature  of  lakes 
exhibits  changes  much  greater  than  those  of  the  ocean.     The  sur- 
face may  freeze  in  winter,  while  in  summer  the  temperature  may 
rise  to  77°.     In  deep  lakes,  at  a  certain  depth,  we  find  a  constant 
temperature  of  about  39°,  this  being  the  temperature  of  water  at 
its  maximum  density.     Since  the  warm  water  of  the  surface  de- 
scends as  fast  as  it  becomes  cooled,  the  surface  of  a  lake  can  not 
freeze  until  the  entire  mass  has  fallen  to  the  temperature  of  39°, 
unless  under  the  influence  of  very  sudden  and  severe  cold. 

In  rivers  the  constant  agitation  of  the  water  tends  to  render 
the  temperature  uniform  throughout.  Hence  the  temperature  at 
the  surface  would  not  change  very  greatly  during  the  year  were 
it  not  for  the  diminished  flow  in  summer,  which  leaves  but  a  thin 
stratum  of  water  to  be  acted  upon  by  the  sun.  During  winter 
congelation  can  not  take  place  until  the  entire  mass  is  cooled  to 
32°,  with  the  exception  perhaps  of  deep  cavities. 

91.  Anchor  Ice. — Ice  sometimes  forms  upon  stones  and  other 
objects  at  the  bottom  of  rivers  when  the  surface  water  is  not 
frozen,  and  this  is  called  anchor  ice.     Such  ice  may  form  under 
the  following  circumstances.     During  a  period  of  severe  cold  the 
water  of  a  river  may  sink  below  32°  from  top  to  bottom  through- 
out, and  the  surface  water  not  congeal,  because  it  is  kept  in  con- 
stant agitation,  while  the  water  at  the  bottom,  being  more  quiet, 
may  congeal.     The  ice  thus  formed  at  the  bottom  forms  a  nucle- 
us about  which  the  congelation  continues  and  extends.     When 
the  ice  becomes  quite  thick,  its  buoyant  force  may  overcome  its 
adhesion  to  objects  at  the  bottom,  and  it  rises  to  the  surface.     A 
slight  elevation  of  temperature,  causing  a  partial  fusion,  may  also 
detach  it  from  the  bottom. 

Anchor  ice  never  forms  at  the  bottom  of  tranquil  water,  be- 
cause congelation  commences  at  the  surface,  while  the  tempera- 
ture of  the  bottom  is  above  32°. 

Sometimes  the  ice  accumulates  upon  the  extreme  edge  of  a 
mill-dam  over  which  water,  a  foot  in  depth,  is  flowing.  Some- 
times the  rack  or  strainer  through  which  the  water  passes  to  the 
water-wheel  of  a  mill  becomes  coated  with  a  mass  of  needle-like 
crystals  of  ice,  so  that  the  flow  of  the  water  is  stopped,  and  the 
mill  can  only  be  kept  running  by  frequently  removing  the  ice 
with  a  rake. 


54  METEOROLOGY. 


CHAPTER  III. 

THE   MOISTURE   OF  THE   AIR. 

92.  How  Water  is  converted  into  Vapor. — If  during  summer  we 
expose  to  the  sun's  rays  a  vessel  containing  water,  we  find  that 
the  water  rapidly  diminishes,  and  in  a  few  days  entirely  disap- 
pears.    The  water  seems  to  have  been  annihilated,  but  in  fact  it 
has  been  converted  into  vapor,  which  is  diffused  through  the  at- 
mosphere.    This  vapor  is  entirely  invisible,  but  by  the  applica- 
tion of  cold  we  may  condense  it,  and  reduce  it  again  to  the  form 
of  water.     Thus  in  summer,  if  we  pour  cold  water  into  a  metallic 
vessel,  we  find  that  the  outside  of  the  vessel,  which  was  previous- 
ly quite  dry,  soon  becomes  covered  with  moisture.     This  moist- 
ure does  not  come  from  the  inside  of  the  vessel.     It  is  simply  the 
vapor  of  the  air,  condensed  by  coming  in  contact  with  a  cold 
surface. 

The  vapor  of  the  air  may  be  condensed  in  a  similar  manner  at 
all  seasons  of  the  year.  The  phenomenon  is  most  frequently  no- 
ticed in  summer,  because  then  the  temperature  of  the  air  rises 
highest  above  that  of  the  water  which  we  are  accustomed  to  use. 
But  at  any  period  of  the  year,  if  the  water  be  not  already  cold 
enough,  by  adding  to  it  ice,  and  if  necessary  salt,  we  may  con- 
dense the  moisture  of  the  air  even  in  the  coldest  weather. 

93.  How  Vapor  is  sustained  in  the  Air. — The  atmosphere  always 
contains  vapor  of  water.     This  vapor  is  not  sustained  in  the  air 
like  water  in  a  sponge,  nor  does  it  float  in  the  air  like  small  par- 
ticles of  dust,  but  it  penetrates  between  the  particles  of  the  per- 
manent gases  which  compose  the  atmosphere,  and  sustains  itself 
precisely  in  the  same  manner  as  they  do.     If  we  exhaust  all  the 
air  from  a  close  vessel,  and  introduce  into  it  a  quantity  of  water, 
a  portion  of  the  water  will  immediately  pass  into  the  state  of  va- 
por, which  will  fill  the  entire  vessel.     Indeed,  with  the  exception 
of  the  facility  with  which  it  is  reduced  to  the  liquid  state,  vapor 
of  water  has  precisely  the  same  properties  as  oxygen  or  nitrogen. 


THE   MOISTURE   OF  THE   AIR.  55 

If  into  a  close  vessel  containing  atmospheric  air  perfectly  dry 
we  introduce  a  quantity  of  water,  vapor  will  be  formed  of  the 
same  tension,  as  if  the  vessel  were  previously  void.  The  only 
difference  will  be  that  in  a  vacuum  the  maximum  tension  of  the 
vapor  will  be  attained  instantly,  while  in  a  vessel  filled  with  gas 
a  certain  time  will  be  required  to  produce  the  same  result. 

94.  Amount  of  Evaporation  Measured. — The  amount  of  evapo- 
ration from  the  earth's  surface  is  measured  by  placing  a  vessel  of 
water  in  the  open  air,  and  determining  the  loss  of  water  from  day 
to  da}'.     The  vessel  usually  employed  for  this  purpose  is  a  cylin- 
der from  six  to  twelve  inches  in  diameter.     It  is  nearly  rilled 
with  water,  the  quantity  having  been   previously  weighed  or 
measured ;  it  is  then  placed  out  of  doors,  freely  exposed  to  the 
action  of  the  atmosphere.     At  the  end  of  twelve  or  twenty-four 
hours  the  water  is  again  measured,  and  the  loss  of  water  shows 

the  amount  of  evaporation  that  has 
taken  place.  If  rain  has  fallen 
between  the  two  observations,  the 
amount  collected  in  the  rain  gauge 
must  be  deducted  from  the  quantity 
in  the  evaporating  gauge.  The  wire 
cage  around  the  gauge,  Fig.  20,  is  to 
prevent  animals,  birds,  etc.,  from 
drinking  the  water. 

From  observations  continued  for  nine  years  at  London,  How- 
ard determined  that  the  average  amount  of  evaporation  was  thirty 
inches  annually,  although  the  annual  fall  of  rain  at  that  place  is 
only  twenty-five  inches. 

95.  Rale  of  Evaporation  Variable. — The  rate  of  evaporation  de- 
pends greatly  upon  the  exposure  of  the  evaporating  dish.     If  the 
vessel  be  freely  exposed  to  the  sun  and  wind,  the  amount  of  evap- 
oration will  be  greater  than  that  which  takes  place  from  the  sur- 
face of  the  earth  ;  but  if  the  vessel  be  very  much  sheltered,  the 
result  will  be  too  small.     The  total  evaporation  from  the  earth's 
surface  in  a  year  must  be  equal  to  the  total  precipitation  in  the 
form  of  rain,  snow,  dew,  etc.;  but  hitherto  the  relative  amount  of 
evaporation  from  the  ocean  and  from  the  land  has  not  been  ac- 
curately determined. 


56 


METEOROLOGY. 


Evaporation  is  accelerated  by  a  brisk  -wind.  The  vapor  which 
rises  from  water  and  pervades  the  surrounding  air,  is  carried  off 
by  a  wind  which  brings  a  fresh  body  of  air  in  contact  with  the 
water. 

96.  Evaporation  at  all  Temperatures. — Evaporation  proceeds  at 
all  temperatures,  even  the  lowest.     If  during  the  coldest  weather 
of  winter  we  weigh  a  lump  of  ice,  and  then  expose  it  in  the  open 
air  on  a  clear  day  upon  the  north  side  of  a  building,  we  soon  find 
that  the  ice  has  lost  weight.     So  also  in  winter  a  large  mass  of 
snow  often  disappears  without  any  appearance  of  liquefaction. 
Evaporation  proceeds,  although  at  a  diminished  rate,  even  when 
the  thermometer  stands  below  zero  of  Fahrenheit. 

HYGROMETERS. 

97.  Any  instrument  adapted  to  measure  the  amount  of  moist- 
ure in  the  air  is  called  a  hygrometer.     An  instrument  which  sim- 
ply indicates  changes  of  humidity  is  called  a  liygroscope.     All  or- 
ganic substances  are  affected  by  moisture,  which  generally  in- 
creases their  dimensions.     Thus  porous  wood  expands  with  an 
increase  of  moisture,  and  contracts  when  deprived  of  moisture. 
A  strip  of  such  wood  may  be  employed  as  a  hygroscope,  but  it  is 
not  sufficiently  sensitive  for  any  useful  purpose.    A  thin  shaving 

Pig.  21.  of  whalebone,  or  a  single  hair,  is  much  more 

sensitive.  A  hair  will  vary  to  the  amount 
of  one  fiftieth  of  its  entire  length  by  simple 
change  of  moisture. 

98.  /Saussure's  Hygrometer.  —  This  instru- 
ment consists  of  a  metallic  frame,  to  the  top 
of  which  is  attached  one  extremity  of  a 
hair,  E  F,  whose  lower  extremity  is  wound 
around  a  small  wheel.  To  the  axis  of  this 
wheel  is  attached  an  index,  C,  whose  ex- 
tremity traverses  a  graduated  arc.  When 
the  moisture  of  the  air  increases,  the  hair 
lengthens  and  the  index  descends ;  when 
the  moisture  decreases,  the  index  rises. 

To  graduate  the  instrument,  we  determine 
two  fixed  points,  viz.,  that  of  saturation  and 


THE  MOISTURE   OF  THE  AIR. 


57 


that  of  extreme  dryness.  To  obtain  the  first  point,  we  place  the 
instrument  under  a  close  vessel  containing  water,  and  mark  the 
position  of  the  index.  For  the  point  of  absolute  dryness,  we  place 
the  instrument  in  a  dry  vessel  containing  quick-lime.  The  in- 
terval between  these  fixed  points  is  divided  into  one  hundred 
parts,  which  are  called  the  degrees  of  the  hygrometer. 

In  Babinet's  hygrometer,  the  variations  in  the  length  of  the 
hair  from  day  to  day  are  measured  by  means  of  a  microscope 
attached  to  the  frame  of  the  instrument. 

The  hair  hygrometer  is  a  very  imperfect  instrument.  It  is 
essential  to  a  perfect  hygrometer  that  two  instruments  made  in- 
dependently in  distant  countries  should  agree  with  each  other. 
But  it  is  found  that  two  instruments  made  with  different  hairs, 
or  with  hairs  differently  prepared,  may  differ  in  their  indications 
five  degrees.  Even  the  same  hygrometer  undergoes  a  gradual 
change,  since  the  length  of  the  hair  increases  from  the  continued 
pressure  of  the  weight  which  it  supports.  This  instrument  is 
therefore  so  unsatisfactory  that  it  has  been  entirely  discarded  in 
scientific  researches. 

99.  Dew-point  Defined. — The  amount  of  vapor  in  the  air  may 
be  measured  with  great  accuracy  by  noting  the  temperature  at 
which  moisture  begins  to  be  condensed  on  a  cold  vessel.  The 
moisture  thus  deposited  is  called  dew,  and  the  temperature  at 
which  this  deposition  begins  is  called  the  dew-point  The  dew- 
point,  then,  may  always  be  determined  by  cooling  a  metallic  ves- 
sel until  dew  begins  to  appear  upon  its  surface,  and  noting  by  a 
thermometer  the  temperature  of  the  vessel.  This  experiment, 
however,  requires  considerable  time,  and  various  contrivances 
Fio.  22.  have  been  proposed  to  facili- 

tate it. 


100.  Baches  Hygrometer.  — 
When  it  is  required  to  determ- 
B  ine  the  dew-point  frequently  at 
short  intervals,  the  following 
apparatus,  invented  by  Profess- 
or Bache,  is  very  convenient. 
A  small  metallic  box,  A,  is  fill- 
ed with  a  mixture  of  salt  and 


58 


METEOROLOGY. 


Fie.  23. 


snow,  by  which  means  its  temperature  is  reduced  to  about  zero. 
From  the  side  of  the  box  projects  a  polished  metallic  bar,  B, 
having  on  its  upper  side  a  groove,  C,  containing  mercury,  in 
which  is  immersed  the  bulb  of  a  thermometer,  D,  which  is  sus- 
pended from  a  support,  E,  so  that  the  thermometer  is  movable 
along  the  groove.  One  end  of  the  bar,  B,  has  a  very  low  temper- 
ature, while  the  other  is  but  little  below  that  of  the  surrounding 
air.  That  portion  of  the  bar  whose  temperature  is  below  the 
dew-point  will  be  covered  with  moisture,  while  the  other  part 
will  be  dry,  and  the  two  portions  will  be  separated  by  a  well- 
defined  bounding  line.  By  placing  the  bulb  of  the  thermometer, 
D,  opposite  to  this  line,  we  may  immediately  determine  the  tem- 
perature of  the  dew-point.  When  only  an  occasional  observa- 
tion of  the  dew-point  is  desired,  this  instrument  is  inconvenient, 
because  it  requires  considerable  time  to  prepare  it  for  experi- 
ment 

101.  DanieWs  Hygrometer. — This  instrument  is  more  convenient 
than  Bache's  when  only  an  occasional  observation  is  to  be  made. 
It  consists  of  two  glass  bulbs,  A  and  B,  about 
three  fourths  of  an  inch  in  diameter,  con- 
nected by  a  small  tube,  which  is  bent  in  two 
places  at  right  angles,  and  the  whole  is  her- 
metically sealed.  The  lower  bulb,  A,  which 
is  made  of  dark-colored  glass,  is  about  half 
filled  with  ether,  and  contains  a  small  ther- 
mometer, T.  The  upper  bulb,  B,  is  covered 
with  a  piece  of  fine  muslin.  If  we  pour 
ether  upon  the  ball  B,  the  ether  will  rapidly 
evaporate  and  produce  cold,  condensing  the 
vapor  of  ether  which  previously  filled  the 
ball  B.  The  ether  in  the  ball  A,  being  relieved  from  the  press- 
ure of  the  vapor  upon  it,  now  rapidly  evaporates,  and  its  tempera- 
ture falls,  as  is  shown  by  the  depression  of  the  thermometer,  T. 
If  this  depression  be  sufficient,  the  vapor  of  the  atmosphere  will 
be  condensed  on  the  outside  of  the  ball,  and  the  state  of  the  ther- 
mometer, T,  at  that  instant  will  indicate  the  dew-point. 

This  instrument  is  ordinarily  very  convenient  for  use,  but  when 
the  atmosphere  is  very  dry  it  requires  ether  of  the  best  quality, 
and  some  dexterity  in  manipulation,  to  obtain  a  deposit  of  dew. 


THE   MOISTURE   OF  THE   AIR. 


59 


Fig.  24. 


102.  Wet-bulb  Thermometer.— The  hy- 
grometer which,  on  account  of  its  con- 
venience, is  now  most  generally  used,  is 
the  wet-bulb  thermometer.  It  consists 
of  a  common  thermometer,  with  its  bulb, 
B,  Fig.  24,  covered  with  a  piece  of  thin 
muslin,  and  kept  constantly  moistened 
with  water  by  means  of  loose  cotton 
threads  communicating  with  a  cup  of 
water,  A.  The  evaporation  of  the  water 
produces  cold,  and  this  thermometer  ha- 
bitually stands  lower  than  a  dry  ther- 
mometer similarly  exposed.  This  de- 
pression strictly  measures  only  the  evap- 
orating power  of  the  air ;  yet,  as  the  lat- 
ter depends  upon  the  amount  of  moist- 
ure present  in  the  air,  the  depression 
of  the  wet-bulb  thermometer  indirectly 
measures  the  humidity  of  the  air. 


103.  Dew-point  deduced  from  the  Wet 
Bulb. — The  difference  between  the  tem- 
perature of  the  air  and  that  of  the  dew-point  is  called  the  comple- 
ment of  the  dew-point.  When  the  air  is  saturated  with  moisture 
the  complement  of  the  dew-point  is  zero. 

From  the  comparison  of  a  great  number  of  observations  with 
Daniell's  hygrometer,  combined  with  simultaneous  observations 
of  the  dry  and  wet  bulb  thermometers,  a  method  has  been  dis- 
covered by  which  the  dew-point  may  be  deduced  from  the  read- 
ings of  the  wet-bulb  thermometer.  The  ratio  of  the  complement 
of  the  dew-point  to  the  depression  of  the  wet-bulb  thermometer 
is  a  variable  one.  When  the  temperature  of  the  air  is  53°,  the 
difference  between  the  readings  of  the  dry  and  wet  bulb  ther- 
mometers is  one  half  the  complement  of  the  dew-point;  at  33°  it 
is  one  third ;  at  26°  it  is  one  sixth ;  and  at  lower  temperatures 
the  ratio  is  still  less.  Table  XXV.,  p.  273,  furnishes  the  factors 
by  which  the  dew-point  may  be  deduced  from  the  readings  of  the 
wet-bulb  thermometer  for  any  temperature  of  the  external  air. 


60 


METEOROLOGY. 


When  the 
dew-point  •< 
is  at 


0.181  inch  in  height. 
.248 
.361 

.518 


104.  Weight  of  Vapor  determined. — The  elastic  force  of  the  va« 
por  present  in  the  air,  that  is,  the  pressure  which  it  exerts,  is  in- 
dicated by  the  dew-point.     Dalton  constructed  a  table  showing 
for  every  degree  of  temperature  the  corresponding  elastic  force  of 
vapor,  and  this  table  has  since  been  brought  to  great  perfection. 

the  pressure 
of  the  vapor 
in  the  air 
will  sustain 
a  column  .733 

of  mercury  [_  1.023 
A  more  extensive  table  is  given  on  page  276.  With  the  as- 
sistance of  such  a  table,  from  the  indications  of  either  of  the  hy- 
grometers already  described,  we  can  deduce  the  elastic  force  of 
the  vapor  present  in  the  air,  and  hence  we  may  determine  its 
weight. 

105.  How  the  Humidity  of  the  Air  is  denoted. — The  character  of 
a  climate,  whether  it  is  to  be  regarded  as  dry  or  humid,  does  not 
depend  simply  upon  the  absolute  amount  of  vapor  present  in  the 
air.     Its  humidity  is  expressed  by  the  ratio  which  the  amount  of 
vapor  actually  present  in  the  air  bears  to  the  amount  which  the 
air  would  contain  if  it  was  saturated.     Thus,  suppose  the  temper- 
ature of  the  air  to  be  60°,  while  the  dew-point  is  50°.     The  press- 
ure of  the  vapor  in  the  air  according  to  the  table  in  Art.  104,  is 
.36  inch ;  but  if  the  atmosphere  were  saturated  with  moisture, 
that  is,  if  the  dew-point  were  60°,  the  pressure  of  the  vapor  would 
be  .52  inch.     Hence  the  air  contains  70  per  cent,  of  the  amount 
of  vapor  which  it  would  contain  if  it  were  saturated,  and  its  hu- 
midity may  be  represented  by  the  number  70.   See  Table  XXVI. 

In  this  manner  we  find  that  at  Philadelphia  the  average  hu- 
midity of  the  air  is  73 ;  that  is,  the  air,  on  an  average,  contains 
about  three  fourths  of  the  vapor  required  for  its  saturation.  At 
St.  Helena,  the  mean  humidity  of  the  air  is  88,  while  at  Denver 
it  is  only  45.  Near  great  bodies  of  water  the  atmosphere  gen- 
erally contains  more  moisture  than  it  does  over  the  interior  of 
continents. 


106.  Extremes  of  Humidity. — In  different  localities  and  at  dif- 
ferent times  we  meet  with  every  variety  of  condition,  from  per- 


THE   MOISTURE   OF  THE  AIR. 


61 


feet  humidity  to  almost  absolute  dryness.  In  ordinary  pleasant 
weather,  the  complement  of  the  dew-point  is  from  10°  to  15°. 
Occasionally,  at  Philadelphia,  it  amounts  to  25°  or  30°,  and  it  has 
been  observed  as  high  as  45°.  In  India,  the  temperature  of  the 
air  has  been  known  to  rise  61°  above  the  dew-point;  and  it  is 
said  that  in  California  the  temperature  has  been  observed  78° 
above  the  dew-point,  in  which  case  the  atmosphere  contained 
only  six  per  cent,  of  the  vapor  required  for  its  saturation. 

107.  Diurnal  Variation  in  amount  of  Vapor. — The  amount  of 
vapor  present  in  the  air  is  subject  to  great  fluctuations,  some  of 
which  are  periodical.     One  of  these  fluctuations  has  a  period  of 
one  day.     At  Philadelphia,  the  amount  of  vapor  present  in  the 
air  is  least  about  an  hour  before  sunrise,  from  which  time  the 
amount  increases  uninterruptedly  until  a  little  before   sunset, 

Fig.  25.  after  which  it  decreases  un- 

40' '     ' '     '     I~~1     '     '     '     '     '     '     interruptedly  until  the  next 

morning.  The  mean  diur- 
nal variation  amounts  to  one 
eighth  part  of  the  average 
amount  of  vapor.  Fig.  25 
shows  the  diurnal  variation  at  Philadelphia,  the  numbers  on  the 
left  indicating,  in  inches  of  mercury,  the  pressure  of  the  vapor  at 
the  hours  given  at  the  bottom  of  the  figure. 

The  cause  of  this  variation  is  obvious.  As  the  sun  rises  and 
the  heat  of  the  day  increases,  more  water  is  evaporated  from  the 
ocean  and  the  moist  earth,  and  the  amount  of  vapor  in  the  air  in- 
creases. During  the  night  a  portion  of  this  vapor  is  condensed 
in  the  form  of  dew  and  hoar-frost ;  that  is,  the  amount  of  vapor 
present  in  the  air  is  least  a  short  time  before  sunrise,  and  greatest 
a  short  time  before  sunset. 

108.  Annual  Variation  in  amount  of  Vapor. — There  is  an  an- 
nual variation  in  the  amount  of  vapor  present  in  the  air.     At 
Philadelphia  the  vapor  present  in  the  air  is  least  in  January  and 
greatest  in  July ;  the  amount  in  July  being  more  than  four  times 
as  great  as  in  January.     This  is  evidently  the  effect  of  the  sun's 
heat  producing  a  more  rapid  evaporation  in  summer  than  in 
winter. 


m't  ->h    4      6      8    10  noon.  2h    4      (5      S     10    m't 


62 


METEOROLOGY. 


Fig.  26. 


109.  Influence  of  Elevation.  —  The  humidity  of  the  air  generally 
diminishes  as  we  rise  above  the  surface  of  the  earth.  From  a 
large  number  of  balloon  ascensions  near  London,  it  has  been 
found  that  when  the  sky  is  clear  there  is  a  slight  increase  of  hu- 
midity until  we  reach  an  elevation  of  3000  feet,  and  afterward  a 
gradual  decrease  to  23,000  feet,  where  the  humidity  is  expressed 
by  16.  When  the  sky  is  overcast  the  increase  of  humidity  up 
to  the  height  of  3000  feet  is  very  slight,  after  which  there  is  gen- 
erally a  decrease,  but  very  irregularly  up  to  23,000  feet.  At  the 
highest  elevations  at  which  observations  have  been  made,  the  air 
has  never  been  found  entirely  free  from  vapor  of  water. 

^  110.  Diurnal  Variation  of  the  Barometer  explained.  —  If  from  the 
entire  pressure  of  the  atmosphere  we  subtract  the  pressure  of  the 
vapor,  the  remainder  will  represent  the  pressure  of  the  gaseous 

portion  of  the  atmosphere. 
At  Philadelphia  the  pres- 
sure  of  the  gaseous  atmos- 
phere is  greatest  about  an 
hour  after  sunrise,  from 

which  time  the  pressure  di- 
't     .  .  . 
mimshes  uninterruptedly  un- 

til about  4  P.M.,  after  which  the  pressure  increases  uninterrupted- 
ly until  the  next  morning,  as  shown  by  the  curve  in  Fig.  26. 

This  fluctuation  appears  to  be  the  effect  of  the  sun's  heat.  As 
the  heat  of  the  day  increases,  the  atmosphere,  becoming  warmed, 
expands  and  rises  to  a  height  greater  than  it  had  during  the 
night  The  upper  portion  flows  off  to  places  where  the  height 
of  the  atmosphere  is  less,  and  the  pressure  of  the  air  at  the  former 
place  is  diminished.  During  the  night  the  temperature  declines, 
the  air  contracts,  its  height  sinks  below  that  which  existed  during 
the  day,  and  air  flows  back  from  regions  where  a  higher  tempera- 
ture prevails,  and  the  pressure  of  the  air  is  increased.  The  pres- 
sure of  the  vapor  and  that  of  the  gaseous  atmosphere  have  each 
but  one  daily  maximum  and  minimum,  but  since  their  maxima 
occur  at  different  hours  of  the  day,  the  sum  of  their  effects  ex- 
hibits two  daily  maxima  and  minima. 

.f  111.  Why  this  Theory  is  unsatisfactory.  —  At  most  places  in  the 
middle  latitudes,  the  pressure  of  the  gaseous  atmosphere  shows 


.GO 
.68 
.56 
.54 
.52 

, 

'  L 



7 

X 

\ 

^" 

\ 

/ 

/ 

\ 

> 

\ 

/ 

\ 

I 

4 

m't    2h    4      68    10  noou.  21i   4      6     8    10  m 


THE  MOISTURE   OF  THE  AIR.  63 

but  one  daily  maximum  and  one  daily  minimum,  as  represented 
in  Fig.  26 ;  but  at  many  places,  especially  within  the  tropics,  after 
subtracting  the  pressure  of  the  vapor  from  the  entire  pressure  of 
the  atmosphere  for  each  hour  of  the  day,  the  remainders  show 
two  daily  maxima,  about  9  A.M.  and  10  P.M.,  and  two  daily 
minima,  about  3  A.M.  and  3  P.M. ;  and  at  some  places  the  night 
minimum  is  nearly  as  decided  as  the  day  minimum.  The  pre- 
ceding theory,  which  was  first  proposed  by  Dove  in  1831,  does 
not  explain  this  double  minimum  in  the  pressure  of  the  gaseous 
atmosphere,  and  must  therefore  be  pronounced  unsatisfactory. 

112.  Espy's  Theory  of  the  Diurnal  Variation. — In  1827,  Espy 
proposed  a  theory  of  the  diurnal  variation  which,  with  some 
modification,  may  be  stated  as  follows:    When  the  sun  rises,  the 
atmosphere  near  the  surface  of  the  earth  becomes  warmed  and 
expanded,  and  the  amount  of  vapor  present  in  the  air  increases. 
An  increased  pressure  is  thus  exerted  upon  the  superincumbent 
air  forcing  it  upward,  but  its  ascent  is  opposed  by  its  inertia. 
The  pressure  at  the  surface  of  the  earth  therefore  increases,  and 
the  maximum  occurs  when  the  temperature  is  increasing  most 
rapidly,  that  is,  about  10  A.M.    The  superincumbent  atmosphere 
having  acquired  momentum  upward,  the  barometer  at  the  surface 
begins  to  fall,  and  continues  to  fall  until  the  heat  of  the  atmos- 
phere begins  to  decline,  which  is  about  4  P.M.     In  consequence 
of  the  loss  of  heat  and  the  condensation  of  vapor  in  the  form  of 
dew,  the  upper  atmosphere  soon  begins  to  sink,  and  acquires  mo- 
mentum downward,  which  causes  the  barometer  at  the  surface  to 
rise,  and  it  attains  a  second  maximum  when  the  temperature  is 
decreasing  most  rapidly,  that  is,  about  10  P.M.     The  elastic  force 
of  the  compressed  air  again  causes  the  superincumbent  atmos- 
phere to  rise,  and  its  inertia  carries  it  beyond  the  point  of  mean 
pressure,  causing  a  second  minimum  about  4  A.M. 

113.  Annual  Variation  of  Pressure  of  the  Gaseous  Atmosphere. — 
Throughout  the  middle  latitudes  of  the  northern  hemisphere,  the 
pressure  of  the  gaseous  atmosphere  is  greatest  in  January,  from 
which  time  it  diminishes  uninterruptedly  until  July,  after  which 
it  increases  uninterruptedly  until  the  succeeding  January;  but 
the  difference  between  the  winter  and  summer  pressures  is  very 
unequal  in  different  countries.     At  Philadelphia  this  difference 


METEOROLOGY. 


Fig.  2T. 


30.2 


30.0 


29.8 


29.6 


29.4 


29.2 


29.0 


28.8 


amounts  to  half  an  inch,  but  throughout  nearly  the  whole  of  Cen- 
tral Asia  the  difference  amounts 
to  an  entire  inch,  and  in  many 
places  is  still  greater,  while  under 
the  equator  the  difference  is  scarce- 
ly appreciable.  Fig.  27  shows  the 
annual  curve  of  pressure  of  the 
gaseous  atmosphere  at  Pekin  in 
China. 

This  fluctuation  in  the  pressure 
of  the  gaseous  atmosphere  is  due 
to  the  influence  of  the  sun's  heat. 
As  the  sun  advances  from  the 
southern  to  the  northern  hemisphere,  the  latter  is  heated  and  its 
atmosphere  expands,  while  the  former  is  cooled  and  its  atmos- 
phere contracts.  The  atmosphere  in  the  northern  hemisphere 
being  thus  rendered  higher  than  in  the  southern,  the  excess  of 
air  in  the  northern  hemisphere  flows  over  to  the  southern.  The 
amount  of  this  effect  depends  upon  the  annual  range  of  the  ther- 
mometer. It  is  most  noticeable  over  the  interior  of  continen 
and  is  scarcely  appreciable  over  the  ocean. 


s 

—j. 

/ 

- 

\ 

t 

\ 

\ 

/ 

\ 

1 

\ 

\ 

V 

\ 

1 

\ 

\ 

k 

J    PMAMJJASOND 


CHAPTER  IY. 

THE   MOTIONS   OF   THE   ATMOSPHERE. 

114.  WIND  is  air  in  motion.  The  movements  of  the  air  are 
proverbially  variable  and  seemingly  capricious,  and  it  has  been 
supposed  that  they  are  not  subject  to  any  law.  We  shall  find, 
however,  that  the  winds  are  subject  to  laws  as  definite  as  those 
of  the  barometer  or  thermometer. 

The  direction  of  the  wind  is  designated  by  the  point  of  the  ho- 
rizon from  which  it  blows.  This  direction  is  commonly  indicated, 
as  in  navigation,  by  the  terms  north,  north  by  east,  north-north- 
east, etc.  If  we  wish  to  indicate  the  direction  with  greater  pre- 
cision, we  may  employ  degrees  of  azimuth,  as  in  astronomy ;  thus 
a  wind  designated  by  N.  13°  E.  comes  from  a  point  13  degrees  to 
the  east  of  north.  Sometimes  it  is  found  convenient  to  designate 


THE   MOTIONS   OF  THE  ATMOSPHERE.  65 

the  direction  by  degrees  of  the  horizon  reckoned  continuously 
from  0°  up  to  360°. 

For  the  purpose  of  investigating  the  laws  which  govern  the 
movements  of  the  atmosphere,  we  require  some  means  of  measur- 
ing both  the  direction  and  velocity  of  the  wind. 

115.  Hoto  to  determine  the  Direction  of  the  Wind. — Any  instru- 
ment for  measuring  the  direction  of  the  wind  near  the  earth's 
surface  is  called  an  anemoscope.  The  simplest  anemoscope  is  the 
common  vane.  In  order  that  the  vane  may  give  reliable  results, 
particular  care  is  required  in  its  construction.  A  vane  usually 
consists  of  a  flat  vertical  plate,  turning  freely  about  an  upright 
spindle.  That  part  of  the  vane  which  is  before  the  spindle,  and 
is  turned  toward  the  wind,  is  called  the  head ;  the  rest  of  the 
vane  is  called  the  tail.  If  a  vane  were  made  in  the  form  of  a 
rectangular  plate  of  uniform  thickness,  and  balanced  upon  its 
centre  of  gravity,  the  action  of  the  wind  upon  the  head  would 
just  neutralize  its  action  upon  the  tail,  and  the  vane  would  have 
no  directive  power.  The  directive  power  of  the  vane  depends 
simply  upon  the  difference  of  the  wind's  action  upon  the  head  and 
tail.  The  tail  should  therefore  present  a  large  amount  of  surface, 
and  the  head  a  small  surface.  Moreover,  in  order  to  maintain 
the  spindle  in  an  upright  position  with  the  least  friction  against 
its  supports,  the  two  ends  of  the  vane  should  exactly  balance 
each  other. 

The  vane  represented  in  Fig.  28  is  designed  to  fulfill  these 
Fig.  28.  conditions.     It 

consists  of  a  rod 
of  iron,  A  B, 
three  fourths  of 
an  inch  in  di- 
ameter, to  one 
end  of  which  is 
attached  a  pine 
board  about 

half  an  inch  thick,  one  foot  wide,  and  eleven  feet  long,  and  bal- 
anced by  a  sphere  of  iron  01  lead,  A,  attached  to  the  other  end 
of  the  rod.  To  give  to  the  instrument  more  steadiness,  the  wood- 
en part  is  made  to  consist  of  two  boards  inclined  at  a  small  angle, 
as  shown  in  the  section  E  G.  The  vane  is  attached  to  an  upright 

E 


66 


METEOROLOGY. 


Fig.  29. 


spindle,  H  K,  which  revolves  freely,  and  the  direction  of  the  wind 
is  measured  by  a  graduated  circle  attached  to  the  spindle. 

116.  Sdf-Tegistering  Anemoscope. — An  anemoscope  may  be  ren- 
dered self -registering  in  the  following  manner:  Place  a  cylin- 
drical vessel  beneath  the  revolving  shaft  C  C', 
which  carries  the  vane  AB,  and  let  it  be  di- 
vided into  a  large  number  of  equal  compart- 
ments, as  shown  in  Fig.  29.  Attach  to  the 
shaft  a  funnel,  D,  filled  with  sand  so  arranged 
that  in  every  position  of  the  funnel  the  sand, 
as  it  flows  out,  shall  fall  into  one  of  the  com- 
partments of  the  cylindrical  vessel.  The 
amount  of  sand  which  collects  in  the  several 
compartments  will  indicate  how  long  the 
vane  is  maintained  in  the  corresponding  po- 
sitions. If  there  are  eighteen  compartments, 
each  will  correspond  to  an  arc  of  twenty  de- 
grees. 

A  second  series  of  compartments  may  be  arranged  in  the  same 
cylindrical  vessel,  and  a  second  funnel,  D',  be  arranged  like  the 
first,  for  the  purpose  of  balancing  the  weight  of  D. 

9  117.  Woltmann's  Anemometer.  —  An  instrument  designed  to 
measure  the  velocity  or  force  of  the  wind  is  called  an  anemometer. 
Woltmann's  anemometer  consists  of  a  small  wind -mill,  to 
whose  axis  is  attached  an  endless  screw,  which  imparts  motion  to 
a  toothed-wheel,  while  the  number  of  revolutions  is  shown  by  an 
index.  An  observation  consists  in  determining  the  number  of 
revolutions  made  in  one  minute,  when  the  sails  are  exposed  to 
the  action  of  the  wind.  In  order  to  deduce  the  wind's  velocity 
from  such  an  observation,  upon  a  calm  day  we  travel  with  the 
apparatus  on  a  carriage  or  a  rail-car,  and  observe  the  number  of 
revolutions  made  in  going  a  known  distance  in  a  given  time. 
The  effect  will  be  the  same  as  if  the  instrument  was  at  rest  and 
the  air  in  motion.  In  this  manner  we  may  construct  a  table 
showing  the  velocity  of  the  wind  corresponding  to  a  given  num- 
ber of  revolutions  of  the  sails  per  minute. 


0118.   WheweWs  Anemometer. — WhewelPs  anemometer,  Fig.  30, 


THE   MOTIONS   OF   THE   ATMOSPHERE. 


67 


V 


Fig-30-  consists  also  of  a 

small  wind- mil], 
with  complete  ap- 
paratus for  regis- 
tering the  total 
effect  of  the 
wind.  The  mill 
is  mounted  upon 
a  vertical  cylin- 
der, C,  about  two 
feet  high,  and 
four  inches  in 
diameter,  and 
around  the  cylin- 
der is  coiled  a 
sheet  of  paper, 
ruled  vertically, 
to  indicate  the 
points  of  the 
compass.  The 
revolution  of  the 
arms  of  the  wind- 
mill, F,  gives  mo- 
tion to  an  end- 
less screw,  wbicli 
causes  a  pencil,  P,  to  descend  along  a  vertical  rod,  and  traces  an 
undulating  line  upon  the  paper  cylinder.  When  the  pencil  has 
reached  the  bottom  of  the  paper  (which  ordinarily  requires  an  in- 
terval of  at  least  twenty-four  hours),  a  new  sheet  of  paper  must 
be  applied  to  the  cylinder,  and  the  pencil  set  back  again  at  the 
top.  The  direction  of  the  wind  is  indicated  by  the  portion  of 
the  sheet  upon  which  th-3  pencil  line  is  traced,  and  its  velocity 
by  the  rate  of  motion  of  the  pencil.  Thus  this  instrument  regis- 
ters the  amount  of  the  wind's  progress  for  every  point  of  the 
compass. 

119.  Robinson's  Anemometer. — Robinson's  anemometer,  Fig.  31, 
consists  of  four  equal  metallic  cups,  A,  B,  C,  D,  in  the  form  of 
hemispheres,  attached  to  two  arms  which  cross  each  other  at  right 
angles,  and  are  supported  so  as  to  turn  freely  about  a  vertical 


METEOROLOGY. 


Tig.  31. 


axis,  E.  The 
base  of  each 
hemispherical 
cup  is  in  a  ver- 
tical position ; 
and  since  the 
action  of  the 
wind  upon  the 
concave  side 
of  one  of  these 
cups  is  greater 

than  its  action  upon  the  convex  side,  a  moderate  breeze  is  suffi- 
cient to  maintain  the  arms  in  continuous  rotation.  Dr.  Robinson 
has  proved  that  (making  no  allowance  for  friction)  the  centre  of 
each  hemisphere  moves  with  one  third  the  velocity  of  the  wind, 
and  thus  this  instrument  measures  directly  the  wind's  velocity. 
The  axis  E  carries  an  endless  screw,  which  gives  motion  to  a 
series  of  wheels  which  register  the  wind's  progress  up  to  1000 
miles. 


Fig. 32. 


•  120.  Osier's 
Anemometer. — 
Osier's  anemom- 
eter, Fig.  32,  reg- 
isters both  the  di- 
rection and  force 
of  the  wind.  It 
consists  of  a  large 
vane,  V,  support- 
ed upon  a  revolv- 
ing spindle.  At- 
tached to  the 
lower  extremity 
of  this  spindle 
is  a  small  pinion 
working  in  a 
rack,  ef,  which 
slides  backward 
and  forward  as 
the  wind  turns 


THE   MOTIONS   OF  THE   ATMOSPHERE. 


69 


the  vane,  and  to  this  rack  is  attached  a  pencil,  A,  which  presses 
against  a  horizontal  sheet  of  paper,  ruled  to  indicate  the  points  of 
the  compass.  This  sheet  of  paper  is  moved  forward  uniformly 
by  clock-work,  C,  at  the  rate  of  half  an  inch  per  hour,  so  that 
while  the  vane  oscillates  to  and  fro,  the  direction  is  registered  on 
the  sheet  of  paper,  which  also  indicates  the  time  at  which  each 
change  took  place.  Fig.  33  shows  the  register  made  by  the  pen- 
cil in  one  day. 


Fte.  33. 


bbs 


121.  How  the  Wind's  Force  is  Measured. — In  order  to  measure 
the  wind's  force,  a  brass  plate,  T,  two  feet  square  is  attached  to 
the  vane,  so  as  always  to  be  presented  perpendicularly  to  the  ac- 
tion of  the  wind.  To  the  back  of  this  plate  is  attached  a  spiral 
spring,  which  is  compressed  by  the  pressure  of  the  wind  against 
the  plate,  and  the  degree  of  compression  of  the  spring  affords  a 
measure  of  the  wind's  force.  By  means  of  a  connecting  wire  this 
square  plate  gives  motion  to  a  second  pencil,  B,  which  at  each  in- 
stant registers  upon  the  same  sheet  the  wind's  force.  At  the  end 
of  twenty-four  hours  a  new  sheet  must  be  applied,  and  thus  each 
sheet  indicates  the  direction  and  force  of  the  wind  for  each  in- 
stant during  a  period  of  twenty-four  hours.  The  undulating  line 
at  bottom  of  Fig.  33  shows  the  register  of  the  wind's  force.  The 
irregular  line  at  the  top  of  the  same  figure  shows  the  amount  of 
rain  registered  by  an  arrangement  not  here  represented. 

122.  How  Velocity  is  deduced  from  Pressure.  —  The  indications 
of  Osier's  anemometer  are  expressed  in  pounds  of  pressure  per 
square  foot.  In  order  to  deduce  from  these  results  the  velocity 
of  the  wind  in  miles  per  hour,  we  require  a  table  showing  the 
velocity  of  the  wind  corresponding  to  different  pressures.  The 
following  table  shows  the  velocity  of  the  wind  in  miles  per  hour, 
corresponding  to  the  pressure  upon  a  square  foot  of  surface,  ac- 
cording to  the  experiments  of  Smeaton. 


70 


METEOROLOGY 


Velocity. 
Miles. 

Pressure. 
Pounds. 

Velocity. 
Miles. 

Pressure.      Velocity. 
Pound*.          Miles. 

Pressure. 
Pounda. 

Velocity. 
Miles. 

Pressure. 
Pounds. 

1 

0.005 

G 

0.177              11 

0.595 

1G 

1.2GO 

2 

.020 

7 

.241 

12       1     0.708 

17 

1.422 

3 

.044 

8 

.315     i 

13 

0.831 

18 

1.594 

4 

.079 

9 

.399 

14 

0.964 

19 

1.776 

5 

.123 

10 

.492     1 

15 

1.107 

20 

1.968 

It  will  be  seen  that  the  wind's  force  varies  as  the  square  of  its 
velocity.  Thus,  when  the  wind's  velocity  is  20  miles  per  hour, 
its  force  is  four  times  as  great  as  that  of  a  wind  blowing  10  miles 
per  hour. 

123.  Wind's  Force  represented  l>y  a  Scale. — When  an  observer 
has  no  anemometer,  he  should  estimate  the  force  of  the  wind  as 
accurately  as  he  is  able,  and  it  is  recommended  to  indicate  the 
wind's  force  by  a  series  of  numbers  from  1  to  10,  according  to  the 
following:  scale : 


Velocity 

Force  in 

Velocity 

Force  in 

No 

Character. 

in  Miles 

Pounds 

No 

Character. 

in  Miles 

Pounds 

per 

per  square 

Hour. 

Foot. 

Hour. 

Foot. 

1 

Just  perceptible. 

2 

0.02 

6 

Very  high  wind. 

45 

10 

2 

Gently  pleasant. 

4 

0.08 

7 

Strong  gale. 

GO 

18 

3 

Pleasant  brisk. 

12J 

0.75 

8 

Violent  gale. 

70 

24 

4 

Very  brisk. 

25 

3.00 

9 

Hurricane. 

80 

31 

5 

High  wind. 

35 

G. 

10 

Most  violent  hurricane. 

100 

49 

The  numbers  in  the  preceding  table  have  been  deduced  from 
a  great  variety  of  experiments.  One  mode  of  experimenting 
consists  in  noting  the  effects  produced  by  a  motion  of  the  ob- 
server at  different  velocities  during  a  clear  day,  as,  for  example, 
upon  a  railway  train.  Another  method  consists  in  measuring 
the  velocity  with  which  light  objects,  like  a  lock  of  cotton,  are 
carried  along  by  the  wind. 

124.  Average  Velocity  of  the  Wind. — Observations  with  ane- 
mometers have  been  made  at  numerous  places  in  Europe  and 
America,  and  at  a  few  stations  in  other  parts  of  the  world.  It  is 
found  that  at  New  York  the  mean  velocity  of  the  wind  during 
the  entire  year  is  eleven  miles  per  hour,  being  least  in  summer, 
when  it  is  nine  miles  per  hour,  and  greatest  in  winter,  when  it  is 
thirteen  miles  per  hour.  At  Chicago  the  average  velocity  of  the 
wind  is  eight  miles  per  hour;  at  New  Orleans  it  is  eight  miles, 
and  at  San  Francisco  it  is  nine  miles  per  hour. 

At  Liverpool,  England,  the  average  velocity  of  the  wind  is 


THE    MOTIONS   OF   THE   ATMOSPHERE. 


71 


thirteen  miles  per  hour;  at  London  it  is  ten  miles  per  hour;  at 
Madras,  India,  it  is  seven  miles  per  hour;  and  at  the  Cape  of 
Good  Hope  it  is  seventeen  miles  per  hour. 

According  to  observations  at  Philadelphia,  the  mean  velocity 
of  the  wind  is  least  about  sunrise.  After  sunrise  the  velocity 
rapidly  increases,  and  becomes  greatest  at  2  P.M.,  after  which  it 

rapidly  declines  till  8  P.M., 
from  which  time  it  changes 
but  little  until  sunrise,  the 
pressure  at  noon  being  fully 
double  that  at  midniht. 


i&  34  rePresents   the   av- 
erage force  of  the  wind  at 

Philadelphia  for  each  hour  of  the  day,  expressed  in  pounds  press- 
ure per  square  foot,  as  shown  on  the  left  of  the  figure. 

125.  Mean  Direction  of  the  Wind.  —  Suppose  a  current  of  air 
coming  from  the  north  passes  the  point 
C,  Fig.  35,  with  a  velocity  u,  continued 
for  a  time  t,  the  amount  of  air  which 
passes  will  be  measured  by  vt.  If  an- 
other current  subsequently  coming  from 
the  south  moves  with  a  velocity  v'  dur- 
ing a  time  t',  the  amount  of  air  which 
passes  will  be  measured  by  v't',  and  the 
resulting  motion  will  be  the  same  as  if  a 
mass  of  air  vt—v't'  passed  the  point  C 

during  the  time  i+t'.  If  then  N  and  S  represent  masses  of  air 
coming  from  the  north  and  south,  the  resulting  motion  will  be 
represented  by  N—  S.  In  like  manner,  if  we  consider  two  winds 
coming  successively  from  the  east  and  west,  the  resulting  motion 
will  be  represented  by  E—  W. 

If  we  represent  these  results  N—  S  and  E—  W  by  the  lines 
C  A,  C  B,  we  may  easily  determine  their  resultant,  C  D.  The 
angle  V  which  it  makes  with  the  meridian  N  S  is  given  by  the 
formula 

DA_CB_E-W 
-~~=' 


A  wind  blowing  from  any  intermediate  point  may  be  resolved 
into  two  others,  one  of  which  coincides  with  a  meridian,  and  the 


72  METEOROLOGY. 

other  is  perpendicular  to  it.  A  wind  from  the  northeast  may  be 
resolved  into  two  others,  one  in  the  direction  of  C  S,  represented 
by  NE  cos.  45°,  and  the  other  in  the  direction  of  C  W,  also  equal 
to  NE  cos.  45°.  A  wind  from  the  northwest,  southeast,  or  south- 
west may  be  resolved  in  a  similar  manner.  If  then  we  consider 
the  winds  from  the  eight  principal  points,  and  regard  motion  from 
N  to  S,  or  from  E  to  W  as  positive,  while  we  regard  motion  from 
S  to  N,  or  from  W  to  E  as  negative,  we  shall  have 

,r    E-W  +  (NE  +  SE-NW-SW)cos.45° 

t  o  n  o*     V  —  __  -  _  -  _ 

-  N—  S  +  (NE  +  NW-SE-SW)  cos.  45°  ' 
The   mean  velocity  of  the   resulting  wind  is   given  by  the 
formula 

CB      E-W-f  (NE  +  SE-NW-SW)  cos.  45° 

—  —  - 


.  -  i7  —  :  -  ^  -  —  . 

sin.  V  sin.  V. 

In  most  meteorological  registers  the  velocity  of  the  wind  is 
not  measured,  or  perhaps  not  even  estimated,  and  we  are  obliged 
to  assume  that  the  average  velocity  of  the  wind  is  the  same  Tor 
all  points  of  the  compass,  in  which  case  N,  S,  E,  etc.,  in  the  pre- 
ceding formula,  represent  simply  the  number  of  times  that  the 
wind  has  blown  from  each  of  these  points. 

The  assumption  that  the  winds  from  the  different  points  of  the 
compass  blow  with  the  same  average  velocity  is  not  entirely  cor- 
rect, and  the  error  which  may  result  from  its  adoption  can  only 
be  determined  by  careful  observations  with  an  anemometer. 

When  the  direction  of  the  winds  is  given  for  more  than  eight 
points  of  the  compass,  we  may  resolve  each  wind  separately  into 
two  rectangular  components  by  means  of  a  traverse  table,  in  the 
same  manner  as  we  resolve  a  traverse  in  navigation.  We  then 
subtract  the  sum  of  all  the  southerly  motions  from  the  sum  of  all 
the  northerly,  and  represent  this  difference  by  C  A.  We  also 
subtract  the  sum  of  all  the  westerly  motions  from  the  sum  of  all 
the  easterly  motions,  and  represent  this  difference  by  C  B.  The 
resulting  direction  will  then  be  given  by  the  equation 

CB 


126.  Wind's  Progress  represented  by  a  Polygon.  —  A  geometrical 
figure  to  represent  the  total  progress  of  the  wind  for  an  entire 
year  may  be  constructed  as  follows:  Draw  the  line  A  B,  Fig.  36, 
to  represent  a  northwest  direction,  and,  assuming  any  convenient 


THE  MOTIONS   OF   THE   ATMOSPHERE.  73 

Fig.  36.  scale,  make  the  length  of  A  B  to  cor- 

respond to  the  northwest  motion  of  the 
wind  for  the  given  time.  Draw  B  C 
to  represent  a  west  direction,  and  make 
its  length  to  correspond  to  the  west 
motion  of  the  wind  upon  the  same  scale 
as  the  preceding.  In  the  same  manner 
draw  CD  for  the  southwest  wind,  and 
so  on  for  each  of  the  other  directions  of  the  wind,  and  suppose 
the  last  line  representing  the  north  winds  reaches  to  I.  Then 
join  A  I,  and  this  line  will  represent  the  direction  and  rate  of  the 
wind's  total  progress  during  the  period  embraced  in  the  observ- 
ations. The  annexed  figure  represents  the  relative  frequency 
of  the  different  winds,  according  to  observations  made  during 
twenty-five  years,  at  about  thirty  academies  in  the  State  of  New 
York. 

By  a  series  of  observations  with  Osier's  anemometer,  it  is  found 
that  at  Philadelphia  the  actual  progress  of  the  wind  is  toward  a 
point  a  little  north  of  east,  and  at  the  average  rate  of  about  four 
miles  per  hour,  or  one  hundred  miles  per  day. 

127.  Observations  of  the  Wind's  Direction. — Although  observa- 
tions with  accurate  anemometers  are  not  very  numerous,  yet  ob- 
servations of  the  common  vane  have  been  made  to  such  an  ex- 
tent as  to  determine  (if  not  the  velocity  of  the  wind)  at  least  its 
average  direction  for  nearly  every  part  of  the  globe.  In  the 
northern  hemisphere  we  have  observations  from  about  six  hund- 
red stations  on  land,  at  which  the  wind's  direction  has  been  re- 
corded for  periods  varying  from  a  few  months  to  more  than  half 
a  century,  and  amounting  in  the  aggregate  to  nearly  three  thou- 
sand years  of  observations.  We  have  also  the  log-books  of  ships 
which  have  penetrated  nearly  every  sea,  and  which  have  been 
collected  at  the  Observatory  of  Washington,  furnishing  more  than 
three  millions  of  observations,  and  embracing  in  the  aggregate  a 
period  of  more  than  three  thousand  years  of  observation.  These 
materials  are  sufficient  to  indicate  with  considerable  precision  the 
average  direction  of  the  wind  for  every  part  of  the  northern  hem- 
isphere, whether  over  the  continents  or  the  ocean,  at  least  as  far 
as  latitude  60°.  Beyond  latitude  60°  observations  are  much  less 
numerous ;  nevertheless,  the  observations  which  we  have  from 


74:  METEOROLOGY. 

this  region  are  pretty  uniform  in  their  indications.  In  the  south- 
ern hemisphere  our  materials  from  the  continents  are  less  abund- 
ant than  in  the  northern  hemisphere,  but  observations  from  the 
ocean  are  very  numerous. 

128.  Three  Systems  of  Winds. — When  we  project  all  these  ob- 
servations upon  a  map  of  the  earth,  we  find  that  the  winds  are 
naturally  divided  into  three  grand  systems. 

1.  The  equatorial  system. 

2.  The  winds  of  the  middle  latitudes;  and, 

3.  The  polar  winds. 

129.  The  Trade  Winds. — Throughout  nearly  the  entire  equato- 
rial region  of  the  globe,  whether  over  the  land  or  on  the  ocean, 
the  winds  preserve  a  remarkable  uniformity ;   on  the  northern 
side  of  the  equator  blowing  almost  invariably  from  some  nprth- 
east  quarter,  and  on  the  southern  side  of  the  equator  from  a 
southeast  quarter.     This  system  of  currents  is  called  the  trade 
winds. 

In  the  Atlantic  Ocean,  the  N.  E.  trades  extend  on  an  average 
from  about  latitude  7°  to  latitude  29°  N.,  while  the  S.  E.  trades 
extend  to  latitude  20°  S.  Between  the  N.  E.  and  S.  E.  trades  is  a 
belt  of  calms  or  variable  winds,  extending  at  different  seasons 
from  150  to  500  miles  in  breadth,  and  the  centre  of  this  belt  is 
about  five  degrees  north  of  the  equator. 

Throughout  the  northern  half  of  the  belt  of  the  N.  E.  trades, 
the  average  direction  of  the  winds  is  from  N.  60°  E. ;  but  near 
latitude  10°  they  veer  more  to  the  east,  and  near  their  southern 
limit  their  direction  is  almost  exactly  east.  The  average  direc- 
tion of  the  S.  E.  trades  is  from  S.  54°  E. 

The  boundaries  of  the  trade  winds  vary  somewhat  with  the 
season  of  the  year.  During  the  summer  they  advance  a  few  de- 
grees farther  toward  the  north,  while  in  winter  they  recede  some- 
what toward  the  south.  In  spring,  the  centre  of  the  belt  of  calms 
is  only  1°  or  2°  north  of  the  equator,  while  in  summer  it  rises  to 
latitude  9°  or  10°. 

130.  Winds  in  the  Middle  Latitudes. — Beyond  the  borders  of  the 
trade  winds  in  either  hemisphere  we  find  the  prevalent  winds  at 
the  earth's  surface  are  from  the  west.     In  the  northern  hemi- 


THE   MOTIONS   OF   THE  ATMOSPHERE. 


75 


sphere  they  blow  from  a  point  a  little  south  of  west,  and  in  the 
southern  hemisphere  from  a  point  a  little  north  of  west.  This 
zone  of  westerly  winds  is  from  25°  to  30°  in  breadth ;  the  west- 
erly motion  being  most  decided  in  the  middle  of  the  belt,  but 
gradually  diminishing  as  we  approach  the  limit  on  either  side. 
Throughout  the  middle  latitudes  of  the  United  States,  the  aver- 
age direction  of  the  wind  is  from  S.  80°  W. ;  and  the  easterly 
winds  are  to  the  westerly  in  about  the  ratio  of  2  to  5. 

So,  also,  between  the  parallels  of  40°  and  60°,  in  the  southern 
hemisphere,  the  prevalent  direction  of  the  surface-winds  is  from 
about  N.  73°  W. ;  and  the  easterly  winds  are  to  the  westerly  as 
1  to  5. 

131.  Direction  of  the  Polar  Winds. — Beyond  the  parallel  of  60° 
the  general  tendency  of  the  winds  is  almost,  without  exception, 
toward  the  equator;  but  in  some  places  the  inclination  is  toward 


the  west,  and  in  others  toward  the  east.     In  the  northern  hemi- 
sphere, beyond  the  parallel  of  60°,  northeast  winds  generally  pre- 


76  METEOROLOGY. 

vail,  but  in  many  districts  the  prevalent  winds  are  from  the 
northwest.  Fig.  37  represents  for  every  latitude  the  prevalent 
direction  of  the  winds  at  the  earth's  surface. 

132.  The  Surface  Winds. — The  winds  here  described  are  the 
winds  which  prevail  at  the  earth's  surface.     They  also  extend  to 
a  considerable  height,  as  is  shown  by  observations  on  the  sum- 
mits of  mountains,  and  by  the  observed  direction  of  the  clouds. 
It  is  believed  that  the   directions  here  given   are  the   average 
directions   of  the  wind,  as  high  as  two  miles  from  the  earth's 
surface,  and  perhaps    somewhat  higher,  including   nearly    (and 
perhaps  fully)  one  half  the  weight  of  the   entire   atmosphere. 
Above  this  height  we  find  an  entirely  different  system  of  winds 
to  prevail. 

133.  Motion  of  the  Upper  half  of  the  Atmosphere. — It  is  evident 
that  over  any  parallel  of  latitude,  the  northerly  motion  of  the  en- 
tire mass  of  the  atmosphere  must  be  exactly  equal  to  its  south- 
erly motion,  otherwise  the  atmosphere  would  be  gradually  with- 
drawn from  certain  portions  of  the  earth,  and  would  accumulate 
over  other  portions.     If,  then,  in  the  equatorial  regions,  we  find 
the  average  motion  of  the  lower  half  of  the  atmosphere  is  toward 
the  equator,  the  average  motion  of  the  upper  half  must  be/rom 
the  equator;  and  we  actually  find  that  in  the  northern  hemi- 
sphere, within  the  region  of  the  trade  winds,  the  upper  half  of 
the  atmosphere  moves  from  the  southwest.     This  is  proved  by 
the  eruptions  of  volcanoes,  and  by  observations  on  the  summits 
of  mountains. 

134.  Evidence  derived  from  Volcanoes. — Within  the  limits  of  the 
trade  winds  are  several  volcanoes,  which  sometimes  eject  ashes  to 
a  great  height,  and  these  ashes  indicate  the  direction  of  the  stra- 
tum of  air  into  which  they  rise.     In  the  West  Indies,  in  latitude 
15°,  on  the  island  of  St.  Vincent,  is  a  volcano  which  in  1812 
emitted  a  vast  quantity  of  ashes.     A  large  mass  of  ashes  fell 
upon  the  island  of  Barbadoes,  which  is  ninety  miles  east  of  St. 
Vincent,  although  between  the  two  islands  the  trade  winds  con- 
tinually blow  with  such  force  that  it  is  only  by  making  a  very 
long  circuit  that  a  ship  can  sail  from  the  latter  to  the  former. 
The  ashes  were  doubtless  transported  by  an  upper  current  blow- 


THE   MOTIONS   OF   THE   ATMOSPHEKE.  77 

ing  in  a  direction  contrary  to  that  which  prevailed  at  the  surface 
of  the  sea. 

A  similar  phenomenon  was  observed  in  January,  1835,  on  the 
great  eruption  of  the  volcano  of  Coseguina,  in  latitude  13°  north, 
on  the  shores  of  the  Pacific.  Some  of  the  ashes  fell  upon  the 
island  of  Jamaica,  at  the  distance  of  700  miles  in  a  direct  line 
northeast  from  the  volcano.  At  the  same  time,  ashes  were  car- 
ried in  the  contrary  direction  westward,  and  fell  upon  a  ship  in 
the  Pacific  more  than  1200  miles  distant. 

135.  Fine  Dust  transported  by  Winds.  —  At  several  places  in 
Southern  Europe,  Lyons,  Genoa,  etc.,  there  has  repeatedly  fallen 
a  fine  dust,  which  was  once  supposed  to  come  from  the  sandy 
plains  of  Africa ;  but  Ehrenberg,  by  examination  with  the  mi- 
croscope, has  shown  that  this  dust  contains  microscopic  organ- 
isms and  dried  infusoria.     Among  them  he  has  found  several 
South  American  species  belonging  to  the  valleys  of  the  Oronoco 
and  the  Amazon,  and  which  have  not  been  found  in  any  other 
part  of  the  world. 

We  must  then  conclude  either  that  this  dust  came  in  part  from 
South  America  through  the  upper  regions  of  the  atmosphere,  or 
these  species  exist  in  some  other  part  of  the  world  hitherto  un- 
discovered. There  is  little  doubt  that  the  former  is  the  true  ex- 
planation, and  we  conclude  that  this  dust  from  South  America  was 
elevated  into  the  upper  regions  of  the  atmosphere,  where  it  met 
a  current  from  the  southwest,  in  which  it  was  transported  a  dis- 
tance of  over  five  thousand  miles  before  it  fell  again  to  the  earth. 

136.  Winds  on  the  Summits  of  Mountains.  —  Observations  on 
the  summits  of  mountains  indicate  the  same  westerly  current  in 
the   upper  regions   of  the  atmosphere.      Upon  the   summit  of 
Mauna  Kea,  on  one  of  the  Sandwich  Islands,  at  the  height  of 
13,95.1  feet,  there  is  uniformly  found  a  blustering  wind  from  the 
southwest,  while  the  regular  trade  wind  from  the  northeast  is 
blowing  at  its  base. 

The  Peak  of  Teneriffe  (12,205  feet  in  elevation)  does  not  reach 
the  limit  of  the  lower  half  of  the  atmosphere,  ytt  the  wind  here 
often  blows  from  the  southwest,  and  the  clouds  over  the  peak 
constantly  move  from  the  southwest  in  a  direction  opposite  to 
the  trade  winds  below.  The  traveler  Bruce  noticed  a  similar 
fact  on  the  mountains  of  Abyssinia. 


r8  METEOROLOGY. 

137.  Upper  Current  in  the  Middle  Latitudes. — Over  the  middle 
latitudes  of  the  northern  hemisphere,  at  a  considerable  elevation, 
we  generally  find  the  wind  blowing  from  a  direction  somewhat 
north  of  west.     This  is  indicated  by  the  following  observations : 

1.  On  the  summit  of  Pike's  Peak,  in  latitude  38°,  and  at  an 
elevation  of  14,216  feet,  the  average  direction  of  the  wind,  as  de- 
duced from  observations  of  five  years,  is  from  a  point  7£  degrees 
north  of  west     At  Fort  Sanders,  another  station  on  the  Rocky 
Mountains,  in  latitude  41°,  and  elevated  7161  feet,  the  average 
direction  of  the  wind  is  also  7  degrees  north  of  west.     On  the 
summit  of  Mt.  Washington,  in  latitude  44°,  and  at  an  elevation 
of  6285  feet,  the  average  direction  of  the  wind,  as  deduced  from 
observations  of  seven  years,  is  from  a  point  38  degrees  north  of 
west     At  St.  Gothard,  in  Switzerland,  in  latitude  46£  degrees, 
and  at  an  elevation  of  6970  feet,  the  average  direction  of  the 
wind  is  from  a  point  7  degrees  north  of  west.     On  one  of  the 
peaks  of  the  Caucasus,  in  latitude  42|-  degrees,  and  at  an  eleva- 
tion of  7071  feet,  the  mean  direction  of  the  wind  is  almost  exactly 
from  the  north. 

2.  In  May,  1783,  the  volcano  Hecla,  in  Iceland,  commenced 
vomiting  out  smoke  and  ashes,  which  continued  for  a  period  of 
more  than  two  months.    This  smoke  rose  to  a  great  height  in  the 
atmosphere,  and  spread  over  nearly  the  whole  of  Europe,  form- 
ing what  was  called  a  dry  fog.    It  appeared  first  in  the  northwest 
part  of  Europe,  gradually  extending  southward  and  eastward  into 
Italy  and  even  into  Syria,  which  seems  to  indicate  that  during 
these  two  months  there  was  an   upper  current  of  atmosphere 
moving  from  the  northwest,  all  the  way  from  Iceland  to  Syria. 

3.  Aeronauts  who  have  ascended  to  the  height  of  10,000  feet 
in  the  middle  latitudes,  usually  find  the  wind  blowing  from  the 
west;  and  if  they  rise  still  higher,  generally  find  the  wind  blow- 
ing from  a  point  somewhat  to  the  north  of  west. 

138.  Upper  Current  in  the  Polar  Regions. — It  is  evident  that  if 
in  the  polar  regions  the  general  progress  of  the  surface  current  is 
toward  the  equator,  there  must  be  an  upper  current  directed  from 
the  equator. 

139.  Entire   System   of  Atmospheric    Circulation.  —  We   hence 
conclude  that  a  section  of  the  atmosphere  made  by  a  meridian 


THE   MOTIONS   OF   THE   ATMOSPHERE. 


79 


would  exhibit  the  system  of 
currents  represented  in  Fig.  38, 
where  N  denotes  the  north  pole, 
S  the  south  pole,  and  E  the 
equator.  Within  the  tropics 
we  find  the  surface  current 
moving  toward  the  equator,  and 
the  upper  current  from  the 
equator.  In  the  middle  lati- 
tudes the  surface  current  is 
moving  from  the  equator,  and 
the  upper  current  toward  the 
equator.  In  the  polar  regions 
the  surface  current  is  from  the 
poles,  and  the  upper  current 
must  therefore  be  toward  the 
poles. 

This  diagram  merely  indi- 
cates whether  the  wind  is  mov- 
ing to  or  from  the  equator. 
Its  easterly  or  westerly  motion 
could  not  be  exhibited  without 
a  modification  of  the  diagram. 
Throughout  the  equatorial  belt  of  winds  in  the  northern  hemi- 
sphere the  surface  current  is  from  the  northeast  and  the  upper 
current  from  the  southwest;  between  the  parallels  of  30°  and  60° 
the  surface  current  is  from  the  southwest  and  the  upper  current 
from  the  northward,  while  beyond  the  parallel  of  60°  the  surface 
current  is  toward  the  equator,  and  the  upper  current  is  from  the 
equator. 

It  is  required  to  explain  this  system  of  atmospheric  circulation. 

140.  Causes  of  the  Winds. — There  are  three  important  causes 
which  contribute  to  the  production  of  wind. 

1.  Unequal  atmospheric  pressure. 

2.  Unequal  specific  gravity  of  the  air;  and, 

3.  The  rotation  of  the  earth. 

Unequal  pressure  tends  to  produce  motion  in  the  atmosphere 
For  conceive  of  two  vertical  columns  of  air  extending  to  the  top 
)f  the  atmosphere,  and  imagine  them  to  be  connected  near  the 


80 


METEOROLOGY. 


Fig.  39. 


earth  by  a  horizontal  tube.  If  the  weight  of  one  column  exceeds, 
that  of  the  other,  the  air  must  flow  from  the  heavier  to  the  light- 
er column,  in  the  same  manner  as  when  water  stands  at  unequal 
heights  in  the  two  arms  of  a  recurved  tube.  The  wind  must 
therefore  blow /row  places  where  the  barometer  is  highest  toward 
places  where  it  is  most  depressed. 

141.  Unequal  Specific  Gravity  of  the  Air.  —  Unequal  specific 
gravity  of  the  air  may  result  from  unequal  temperature  or  from 
unequal  humidity. 

Let  ACB/Fig.  39,  represent 
an  extended  region  of  country, 
a  portion  of  which,  near  C,  is 
covered  with  sand,  and  becomes 
intensely  heated  by  the  rays  of 
the  sun,  while  at  A  and  B  the 
earth  is  covered  with  vegeta- 
tion. The  air  which  rests  upon 
C,  being  more  expanded  than 
the  surrounding  air,  rises,  and 
its  place  is  supplied  by  air  flow- 
ing horizontally  from  A  and  B 
in  the  direction  of  the  arrows.  At  the  same  time,  tne  column  of 
air,  DEFG,  being  expanded,  and  rising  above  the  surrounding 
atmosphere,  overflows  on  each  side  in  the  direction  of  the  arrows 
HK,  producing  upper  currents  moving  in  a  direction  contrary  to 
the  winds  at  A  and  B,  and  at  a  certain  distance  give  rise  to  de- 
scending currents  to  supply  the  place  of  the  air  which  near  the 
earth's  surface  flows  toward  the  heated  region. 

The  motion  here  described  may  be  illustrated  by  the  following 
experiment :  If  in  winter  we  partially  open  a  door  communica- 
ting between  a  hot  and  a  cold  room,  and  hold  a  lighted  candle 
near  the  top  of  the  crevice,  the  flame  will  be  bent  outward  from 
the  warm  room,  indicating  a  current  of  air  from  the  hot  to  the 
cold  room ;  but  if  we  hold  the  candle  near  the  bottom  of  the 
crevice,  the  flame  will  be  bent  inward,  indicating  a  current  from 
the  cold  to  the  hot  room.  We  thus  discover  that  the  air  flows 
out  at  the  top  of  the  heated  room,  while  the  cold  air  enters  near 
the  floor.  In  a  similar  manner,  the  unequal  warmth  of  the  earth's 
surface  gives  rise  to  currents  of  air  of  immense  extent,  the  denser 
nir  flowing  under  and  displacing  the  lighter. 


THE  MOTIONS  OF  THE  ATMOSPHEKE.  81 

The  specific  gravity  of  the  vapor  of  water  is  only  about  two 
thirds  that  of  dry  air  at  the  same  temperature  and  pressure; 
and  since  it  requires  time  for  vapor  to  diffuse  itself  through  the 
atmosphere,  an  excess  of  aqueous  vapor  must  give  rise  to  cur 
rents  in  the  atmosphere  in  the  same  manner  as  inequality  of 
temperature. 

Even  then,  though  the  barometer  may  every  where  indicate  the 
same  pressure,  the  wind  at  the  surface  of  the  earth  will  tend  from 
the  colder  to  the  warmer  region,  from  the  place  where  the  atmos- 
phere contains  tne  least  vapor  to  that  where  there  is  the  most 
vapor. 

142.  Mode  of  Propagation  of  Winds. — The  wind  is  first  noticed 
near  the  heated  column  of  air,  and  gradually  extends  to  a  greater 
distance  from  it.     As  the  air  moves  from  A  and  B  toward  the 
ascending  column  DEFG,  the  air  at  A  and  B  is  rarefied,  and 
this  rarefaction  is  communicated  to  the  more  distant  air,  and  so 
on ;  that  is,  the  wind  is  propagated  in  a  direction  contrary  to  that 
in  which  it  blows.     Winds  thus  propagated  are  called  winds  of 
aspiration.     Winds  which  are  propagated  in  the  same  direction 
as  that  in  which  they  blow  are  called  winds  of  impulsion.     Ex- 
amples of  both  of  these  classes  of  winds  are  found  in  all  great 
storms,  as  will  be  shown  in  Chapter  VI. 

143.  Rotation  of  the  Earth. — The  rotation  of  the  earth  would 
alone  produce  no  permanent  wind,  because,  if  there  were  no  other 
disturbing  causes,  the  atmosphere  would,  by  friction  upon  thfi 
earth's  surface,  soon  acquire  the  same  velocity  of  rotation  as  that 
of  the  portion  of  the  earth  upon  which  it  rested ;  but  the  earth's 
rotation  materially  modifies  the  operation  of  other  disturbing 
causes. 

Since  the  earth  is  nearly  a  sphere,  rotating  upon  its  axis  once 
in  twenty-four  hours,  the  velocity  of  rotation  of  different  parallels 
of  latitude  is  very  different. 

In  latitude    0°  the  velocity  eastward  is  1036  miles  per  hour. 

it         it  150     u  <(  it  (i    J000        "         "         " 

((  U  QA°      ((  It  It  It        00*7  ((  ((  « 

«       «        45°    tt         tt  n          it     732      tt      tt      » 

41  It  QQ°      tt  tt  It  It        g^g  «  U  <( 

u         u  ITKO     u  tt  tt  u      268        "         "         " 

F 


82  METEOROLOGY. 

144.  Relative  Motion  resulting  from  this  Rotation. — If  a  mass  of 
quiescent  air  from  the  parallel  of  30°  could  be  suddenly  trans- 
ported to  the  parallel  of  15°,  it  would  have  an  easterly  motion 
103  miles  per  hour  less  than  that  of  the  parallel  arrived  at ;  that 
is,  it  would  have  a  relative  motion  westward  of  103  miles  per 
hour.     So  also,  if  a  mass  of  air  from  the  parallel  of  15°  could  be 
suddenly  transported  to  the  parallel  of  30°,  it  would  have  an  east- 
erly motion  103  miles  per  hour  greater  than  that  of  the  parallel 
arrived  at.     That  is,  in  general,  if  air  is  transported  from  the 
equator  toward  the  poles,  it  will  have  a  relative  motion  eastward; 
and  if  air  is  transferred  from  a  higher  latitude  toward  the  equa- 
tor, it  will  have  a  relative  motion  westward.    See  Art.  263. 

145.  Surface  Winds  in  the  Equatorial  Regions. — We  have  seen, 
Art.  20,  that  near  the  parallel  of  32°  the  mean  height  of  the  ba- 
rometer is  greater  than  in  any  other  part  of  the  earth,  and  is  .283 
inch  greater  than  it  is  near  the  equator.     Also,  the  mean  temper- 
ature of  the  surface  air  at  the  equator  is  about  12°  higher  than  it 
is  over  the  parallel  of  32°.     For  both  of  these  reasons,  the  air 
must  tend  from  the  parallel  of  32°  toward  the  equator;  and  if  no 
other  force  acted  upon  it,  the  motion  of  the  air  in  either  hemi- 
sphere would  be  along  a  meridian  toward  the   equator.     But 
while  the  air  from  the  parallel  of  32°  in  the  northern  hemisphere 
flows  toward  the  equator,  it  retains  the  easterly  motion  of  the 
place  from  which  it  started,  and  in  its  progress  southward  reaches 
in  succession  parallels  moving  eastward  more  rapidly  than  itself. 
It  therefore  drags  continually  behind  ;  that  is,  its  motion  with 
reference  to  the  earth's  surface  is  toward  the  west.     Under  the 
action  of  these  two  forces  the  progress  of  the  air  is  toward  the 
southwest,  and  the  exact  path  described  will  depend  upon  the 
relative  magnitude  of  the  southerly  and  westerly  motions. 

A  similar  result  must  be  produced  on  the  south  side  of  the 
equator,  and  thus  originates  a  system  of  currents  flowing  from 
the  northeast  in  the  northern  hemisphere,  and  from  the  southeast 
in  the  southern  hemisphere. 

146.  Upper  Current  in  the  Equatorial  Regions. — The  mean  tem- 
perature of  the  surface  air  at  the  equator  is  considerably  higher 
than  it  is  over  the  parallel  of  32°,  while  near  the  upper  limit  of 
the  atmosphere  the  temperature  must  be  nearly  the  same  in  all 


THE   MOTIONS   OF  THE   ATMOSPHERE.  83 

latitudes.  Now  air  is  expanded  by  heat  to  the  amount  of  ^-rrth 
part  of  its  bulk  for  each  degree  of  the  thermometer.  The  atmos- 
phere over  the  equator  must  therefore  rise  somewhat  higher  than 
it  does  over  the  parallel  of  32°,  notwithstanding  the  difference  in 
the  height  of  the  barometer.  If  the  earth  were  at  rest,  the  air 
thus  expanded  at  the  equator  would  flow  over  at  the  top,  and 
descend  as  along  an  inclined  plane  toward  the  middle  latitudes. 
But  while  in  the  northern  hemisphere  an  upper  current  flows  to- 
ward the  poles,  it  crosses  in  succession  parallels  of  latitude  whose 
easterly  motion  is  less  than  its  own  ;  and  since  it  retains  the  east- 
erly motion  which  it  had  at  the  equator,  it  has  a  relative  motion 
from  the  west,  which,  combined  with  the  first  northerly  motion, 
carries  it  toward  the  northeast.  Thus  above  the  northeast  trade 
winds  we  find  an  upper  current  moving  from  the  southwest 

For  a  similar  reason,  in  the  southern  hemisphere,  above  the 
southeast  trades,  the  upper  current  moves  from  the  northwest. 

147.  The  Surface  Wind  in  the  Middle  Latitudes. — Over  the  par- 
allel of  32°  the  mean  pressure  of  the  air  is  .558  inch  greater  than 
over  the  parallel  of  64°,  and  therefore  at  the  earth's  surface  the 
air  tends  from  the  parallel  of  32°  toward  the  pole.     The  air  in 
latitude  32°  is  indeed  warmer,  and  therefore  lighter  than  it  is  near 
the  poles,  and  this  creates  a  tendency  of  the  surface  current  from 
the  poles  toward  the  equator ;  but  the  effect  of  the  increased 
pressure  of  the  air  near  the  parallel  of  32°  is  greater  than  that 
of  its  diminished  density,  and  the  air  actually  moves  toward  the 
poles. 

But,  while  in  the  northern  hemisphere  the  air  from  the  parallel 
of  32°  moves  northward,  it  crosses  successively  parallels  of  lati- 
tude whose  easterly  motion  is  less  than  its  own ;  and  since  it  re- 
tains the  easterly  motion  which  it  had  at  starting,  it  has  a  relative 
motion  from  the  west,  which,  combined  with  the  first  northerly 
motion,  carries  it  toward  the  northeast.  Thus  throughout  the 
middle  latitudes  of  the  northern  hemisphere  the  prevalent  mo- 
tion of  the  lower  portion  of  the  atmosphere  is  from  the  southwest, 
and,  for  like  reasons,  in  the  southern  hemisphere  the  lower  por- 
tion of  the  atmosphere  moves  from  the  northwest. 

148.  The  Surface  Wind  in  the  Polar  Regions. — It  is  believed  that 
in  the  neighborhood  of  the  north  pole  the  mean  pressure  of  the 


84:  METEOROLOGY. 

atmosphere  is  somewhat  greater  than  it  is  near  the  parallel  of 
64°,  and,  since  the  air  is  colder,  it  has  a  greater  density.  Both 
causes,  therefore,  conspire  to  impel  the  air  toward  the  lower  lati- 
tudes ;  and  this  force,  combined  with  the  effect  of  the  earth's  ro- 
tation, produces  a  northeast  wind  within  the  Arctic  circle.  But 
little  is  known  of  the  winds  within  the  Antarctic  circle. 

149.  Ascending  Current  near  the  Parallel  of  64°. — We  thus  find 
that  in  the  northern  hemisphere  the  surface  wind  from  each  side 
of  the  parallel  of  64°  blows  toward  that  parallel.     This  wind  here 
rises  from  the  earth's  surface  as  it  does  near  the  equator,  and  be- 
comes an  upper  current  receding  on  either  side  from  the  parallel 
of  64° ;  but  this  upper  current  will  not  continue  its  course  ex- 
actly in  the  direction  of  a  meridian.     As  the  air  advances  south- 
ward, it  crosses  successively  parallels  whose  easterly  motion  is 
more  and  more  rapid,  so  that,  after  some  time,  the  direction  of 
the  upper  current  should  be  from  the  northeast     It  must  not  be 
inferred  from  the  reasoning  in  Arts.  143-9  that  a  current  of  air 
moving  in  an  east  and  west  direction  would  experience  no  de- 
viation in  consequence  of' the  earth's  rotation.     It  is  shown  in 
Art.  263  that  the  rotation  of  the  earth  exerts  the  same  deflecting 
force  upon  currents  of  air  moving  in  any  direction. 

150.  Cause  of  the  High  Barometer  near  the  Parallel  o/"320. — If 
the  pressure  of  the  barometer  were  the  same  at  all  points  of  the 
earth's  surface,  in  consequence  of  the  greater  heat  of  the  equato- 
rial regions,  there  would  be  a  general  tendency  of  the  surface  air 
toward  the  equator,  and  of  the  upper  air/rom  the  equator.     This 
upper  current  could  not,  however,  proceed  on  uninterruptedly  to 
the  pole,  because  the  meridians  converge,  and  their  distance  from 
each  other  continually  diminishes,  until  they  all  meet  at  the  poles. 
As  the  upper  current  of  air  recedes  from  the  equator,  it  crosses  suc- 
cessively parallels  of  less  and  less  circumference,  by  which  means 
the  atmosphere  is  forced  up  to  a  corresponding  height,  and  its 
pressure  upon  the  earth's  surface  thereby  increased.     In  latitude 
32°,  the  distance  between  the  meridians  is  nearly  one  sixth  less 
than  it  is  at  the  equator.     This  increased  pressure  of  the  air  in 
the  middle  latitudes  arrests  the  farther  progress  of  the  polar  cur- 
rent, and  a  calm  ensues.     The  upper  air  descends  to  the  earth's 
surface,  and  joins  the  surface  current  toward  the  equator,  where 
it  again  ascends,  and  thus  maintains  a  perpetual  circulation. 


THE   MOTIONS   OF  THE   ATMOSPHERE. 


85 


Pig.  40. 


The  high  barometer  near  the  parallel  of  32°  forces  a  surface 
current  northward  in  opposition  to  the  increased  density  of  the 
air  arising  from  a  diminished  temperature.  Beyond  the  parallel 

of  64°  the  latter  tendency  is  stronger 
than  the  former,  and  the  surface  cur- 
rent is  from  the  poles. 

The  cause  of  the  low  barometer 
near  the  parallel  of  64°  will  be  con- 
sidered in  Chapter  VI.,  page  147. 

There  is  some  reason  for  supposing 
that  in  the  most  elevated  regions  of 
the  atmosphere,  where  the  atmosphere 
has  nearly  reached  its  limit  of  tenui- 
ty, the  current  over  the  equatorial 
regions,  instead  of  descending  to  the 
earth's  surface  near  latitude  32°,  may 
continue  on  the  same  course  over  the 
middle  latitudes  to  the  polar  regions, 
as  represented  in  Figure  40 ;  but  the 
principal  mass  of  the  atmosphere  is 
believed  to  circulate  as  represented  in 
Fig.  38. 

151.  The  Monsoons. — In  mid-ocean  the  direction  of  the  trade 
winds  is  quite  uniform,  but  in  the  neighborhood  of  the  conti- 
nents great  irregularities  are  observed.     The  most  remarkable  of 
these  occur  in  the  Indian  Ocean,  and  are  known  by  the  name  of 
monsoons.     During  the  cooler  half  of  the  year,  from  October  to 
March,  the  regular  trade  winds  prevail  here  as  in  other  parts  of 
the  northern  hemisphere ;  but  during  the  warmer  half  of  the 
year,  from  April  to  September,  the  prevalent  wind  blows  in  the 
contrary  direction,  viz.,  from  the  southwest. 

152.  Cause  of  the  Monsoons. — This  change  of  wind  results  from 
the  influence  of  the  sun's  heat  upon  the  continent  of  Asia.     In 
summer,  the  southern  part  of  Asia  becomes  warmer  than  the  In- 
dian Ocean  near  the  equator,  and  the  cooler  air  from  the  ocean 
rushes  northward  toward  the  land  to  displace  the  heated  air. 
This  air,  coming  from  a  lower  latitude,  has  an  excess  of  motion 
toward  the  east,  which,  combined  with  the  motion  from  the  south 


86  METEOROLOGY. 

due  to  the  influence  of  heat,  produces  a  wind  from  the  southwest. 
This  southwest  wind  sweeps  over  the  high  range  of  mountains 
north  of  Hindostan,  by  which  means  its  vapor  is  condensed,  form- 
ing excessive  rains,  by  which  means  a  vast  amount  of  latent  heat 
is  liberated,  and  the  surrounding  air  is  still  more  expanded,  thus 
adding  to  the  force  of  the  previous  southwest  current  in  a  man- 
ner which  is  explained  in  Chapter  VI.,  page  147. 

During  winter  the  Indian  Ocean  is  warmer  than  the  southern 
part  of  Asia,  and  the  air  from  the  land  flows  toward  the  equator, 
producing  the  usual  northeast  trade  wind. 

153.  Influence  of  the  Seasons. — Similar  phenomena  are  noticed 
in  every  part  of  the  world  near  the  coasts  of  extensive  conti- 
nents.    The  continents  being  colder  than  the  ocean  during  the 
winter  and  warmer  during  the  summer,  the  winds  tend  from  the 
land  in  winter  and  from  the  sea  in  summer.    Along  the  Atlantic 
coast  of  North  America,  from  Nova  Scotia  to  Virginia,  the  prev- 
alent direction  of  the  wind  is  from  the  northwest  in  winter  and 
from  the  southwest  in  summer.     As  we  proceed  southward,  the 
difference  between  the  directions  of  the  wind  in  winter  and  sum- 
mer increases.     From  North  Carolina  to  Florida  the  average 
change  in  the  direction  of  the  winds  from  winter  to  summer 
amounts  to  120  degrees;  and  along  the  northern  shore  of  the 
Gulf  of  Mexico  the  average  change  is  about  180  degrees;  the 
winds  being  generally  from  the  north  in  winter  and  from  the 
south  in  summer,  constituting  well-marked  monsoons.    A  similar 
change,  but  considerably  less  in  amount,  is  observed  generally 
throughout  the  interior  of  the  United  States.     At  St  Louis  in 
winter  the  prevalent  winds  are  from  the  west,  and  in  summer 
from  the  south.     At  Milwaukee  in  winter  the  prevalent  winds 
are  from  the  west,  and  in  summer  from  the  south -south  west. 

On  the  Pacific  coast  of  the  United  States,  there  is  also  a  change 
in  the  prevalent  winds  from  winter  to  summer.  At  San  Fran- 
cisco in  summer  the  prevalent  winds  are  from  the  southwest,  but 
in  winter  they  are  from  the  west-northwest. 

154.  Land  and  Sea  Breezes. — The  diurnal  change  of  tempera- 
ture has  a  sensible  effect  upon  the  direction  of  the  wind.     This 
is  seen  in  land  and  sea  breezes  which  prevail  on  the  coasts  of 
continents  and  islands,  particularly  in  tropical  countries.    During 


THE   MOTIONS  OF  THE   ATMOSPHERE.  87 

the  day  the  land  is  heated  more  rapidly  than  the  sea,  and  during 
the  night  it  is  more  rapidly  cooled.  In  the  morning,  the  air  in 
immediate  contact  with  the  land,  being  heated,  is  displaced  by  the 
cooler  air  in  contact  with  the  sea,  and  thus  arises  a  breeze  from 
the  sea  to  the  land.  In  summer  this  breeze  usually  springs  up 
soon  after  8  A.M.,  and  attains  its  greatest  intensity  about  the  time 
of  highest  temperature.  About  sunset  the  breeze  ceases  entirely. 

During  the  night  the  land  becomes  colder  than  the  sea,  and  a 
breeze  springs  up  from  the  land  to  the  sea,  which  attains  its 
greatest  force  about  the  time  of  lowest  temperature.  This  breeze 
extends  only  to  a  short  distance  from  the  coast. 

If  no  other  cause  operates  to  produce  a  wind,  the  direction  of 
the  land  and  sea  breeze  will  be  perpendicular  to  the  coast ;  but 
if  some  other  cause  operates  at  the  same  time,  the  actual  direction 
of  the  wind  will  be  such  as  results  from  the  composition  of  the 
two  forces. 

155.  Sea  Breeze  in  the  Temperate  Zones. — In  the  temperate  zones 
the  diurnal  change  of  temperature  produces  a  sensible  effect  in 
modifying  the  direction  of  the  prevalent  wind,  and  sometimes 
entirely  reverses  its  direction.     At  New  Haven  the  average  di- 
rection of  the  wind  throughout  the  year  is  20°  more  southerly 
at  noon  than  it  is  at  sunrise,  and  from  March  to  September  the 
average  change  amounts  to  35°.     This  effect  is  so  uniform  that 
sometimes  every  day.  without  exception  for  an  entire  month,  the 
wind  at  noon  is  more  southerly  than  it  was  at  sunrise.      Fre- 
quently the  change  amounts  to  180°,  the  wind  blowing  from  the 
north  at  sunrise  and  from  the  south  at  noon,  and  this  phenome- 
non is  rarely  observed  except  during  clear  and  pleasant  weather, 
indicating  that  the  change  of  wind  is  not  the  result  of  a  great 
storm  in  progress. 

156.  Temperature  of  the  Wind. — The  temperature  of  the  wind 
depends  upon  the  quarter  from  which  it  blows  and  the  countries 
which  it  has  traversed.     Generally  in  the  northern  hemisphere 
the  winds  from  the  south  are  warm,  while  those  from  the  north 
are  cold ;  but  the  precise  point  of  the  horizon  corresponding  to 
the  greatest  heat  and  the  greatest  cold  varies  considerably.     The 
following  table  shows  how  much  the  temperature  of  each  wind  is 
above  or  below  the  mean  at  New  Haven,  as  deduced  from  a  com- 
parison of  several  years  of  observations. 


88 


METEOROLOGY. 


Wind. 

Temperature. 

Wind. 

Temperature. 

N. 
N.E. 
E 
S.E. 

-2°.  7 
-0   .6 
40  .5 
+  1  .2 

S. 
S.W. 

w. 

N.W. 

+  3°.  2 
+  4  .0 
-1  .1 
-4  .5 

Fig.  41. 


+  4 
+  2 
0 
-2 
-4 

1 

X 

\ 

•  

_^ 

-^ 



S 

/ 



\ 

,/ 

E    SE     S    SW   W  N\V   N 


If  we  represent  these  differences  of  temperature  by  the  ordi- 

nates  of  a  curve,  we  shall  obtain  a 
diagram  like  Fig.  41,  which  indicates 
that  at  New  Haven  the  highest  tem- 
perature accompanies  a  wind  from 
south  33°  west,  and  the  lowest  tem- 
perature corresponds  to  a  wind  from 
north  40°  west,  the  mean  difference  in  the  temperature  of  these 
two  winds  being  8°.7. 

In  many  parts  of  Europe  the  coldest  wind  comes  from  a  quar- 
ter somewhat  east  of  north,  and  the  hottest  wind  generally  comes 
from  a  point  a  little  west  of  south. 

157.  Hoi  Wi'ncfc  o/Zteser&. — On  the  deserts  of  Africa  and  Ara- 
bia there  sometimes  prevails  a  wind  extremely  dry  and  intensely 
hot,  which  raises  clouds  of  sand,  and  transports  it  to  a  great  dis- 
tance.    These  winds  are  known  by  the  name  of  simoon,  harmal 
ton,  etc.,  according  to  their  locality.     Plants  are  withered  by  thi-i 
wind;  men  and  animals  suffer  intensely  from  the  heat  and  dry 
ness  of  the  air;  and  entire  caravans  have  been  buried  in  the  drift 
ing  sand.     This  dust  is  sometimes  transported  across  the  Medi 
terranean  into  Spain,  Sicily,  and  Italy,  where  the  wind  which 
brings  it  is  known  by  the  name  of  sirocco.     In  Sicily,  during  its 
continuance,  the  thermometer  sometimes  rises  to  110  degrees  in 
the  shade. 

158.  Dry  Winds  from  Mountains. — In  mountainous  countries 
the  winds  from  certain  quarters  are  celebrated  for  their  dryness. 
The  westerly  winds  which  cross  the  range  of  the  Sierra  Nevadas, 
being  cooled  by  elevation,  deposit  most  of  their  moisture  upon 
the  west  side  of  the  mountains,  and  when  they  descend  upon  the 
eastern  side  they  are  extremely  dry.    These  dry  winds  lose  much 
of  their  remaining  moisture  in  passing  over  the  Rocky  Moun- 
tains, and  but  little  rain  falls  along  the  eastern  margin  of  these 
mountains. 


PRECIPITATION  OF  THE  VAPOR  OF  THE  AIR.       89 

The  high  mountains  of  South  America  produce  effects  still 
more  remarkable.     In  Peru,  between  two  great  chains  of  the 
Fig.  42.  Andes,  Fig.  42,  in  latitude  16°  S.,  at 

the  height  of  13,000  feet,  is  a  deso- 
late table-land  called  the  Punos,  ex- 
tending about  500  miles  in  length 
by  100  in  breadth.  The  trade  wind, 
by  passing  over  the  eastern  chain 
of  mountains,  is  reduced  to  a  very 
low  temperature,  and  nearly  all  its 

vapor  is  condensed  in  the  form  of  rain  or  snow.  When  the 
air  descends  upon  the  western  side  of  the  mountain  it  is  ex- 
tremely dry,  so  that  the  bodies  of  dead  animals  exposed  to  it  are 
dried  up  like  mummies,  without  any  signs  of  putrefaction.  Pres- 
cott  states  that  the  ancient  Peruvians  preserved  the  bodies  of 
their  dead  for  ages  by  simply  exposing  them  to  the  dry  air  of 
the  mountain. 


CHAPTER  V. 

PRECIPITATION  OF  THE  VAPOR  OF  THE  AIR. 

SECTION  I. 

DEW. 

159.  Effect  of  Radiation  of  Heat. — All  bodies  on  the  surface  of 
the  earth  send  out  rays  of  heat  toward  the  sky,  and  when  they 
radiate  more  heat  than  they  receive,  their  temperature  falls  below 
that  of  the  surrounding  air.  In  order  to  study  these  effects,  we 
place  a  number  of  thermometers  upon  the  ground  on  substances 
of  different  kinds,  and  suspend  other  thermometers  in  the  air  at 
various  elevations,  and  compare  the  readings  of  these  thermome- 
ters simultaneously  at  all  hours  of  the  day  and  night.  Very  care- 
ful observations  of  this  kind  were  made  at  Greenwich,  England, 
for  two  years,  and  it  was  found  that  a  thermometer  placed  on 
grass  fully  exposed  to  the  sky  frequently  sinks  ten  degrees  below 
a  thermometer  suspended  four  feet  from  the  ground.  On  nine 
nights  the  difference  of  temperature  was  more  than  15°,  and  in 
one  instance  a  thermometer  placed  on  raw  wool  sunk  28°. 5  below 
one  suspended  eight  feet  from  the  ground. 


90  METEOROLOGY. 

Radiation  of  heat  from  the  earth  to  the  sky  takes  place  at  all 
times,  both  day  and  night,  and  in  all  states  of  the  sky.  Generally, 
when  the  sun  is  above  the  horizon,  the  heat  received  from  it  by 
the  earth  exceeds  that  which  is  radiated  from  the  earth.  Some- 
times, however,  in  places  sheltered  from  the  sun,  but  open  to  a 
considerable  portion  of  the  sky,  the  amount  of  heat  radiated  ex- 
ceeds that  received  from  the  sun  and  all  other  sources,  so  that 
grass  may  continue  colder  than  the  air  during  the  day  as  well  as 
the  night.  This  difference  at  midday  has  been  known  to  amount 
to  ten  degrees. 

160.  Effect  of  Partial  Exposure  to  the  Sky. — Whatever  impairs 
the  free  exposure  of  an  object  to  the  sky  causes  its  temperature 
to  decrease  less  than  it  would  if  the  exposure  was  complete.    This 
effect  is  produced  by  spreading  a  sheet  of  cloth  over  the  ground, 
even  though  it  be  at  a  considerable  elevation.     The  thinnest  cam- 
bric handkerchief  produces  a  decided  effect.    Trees  and  buildings, 
and  whatever  conceals  a  part  of  the  sky,  diminish  the  effect  of 
the  radiation  of  heat  from  the  surface  of  the  earth. 

Clouds  produce  the  same  effect  as  an  artificial  covering.    From 
an  average  of  all  the  Greenwich  observations,  it  was  found  that  a 
thermometer  placed  on  grass  fully  exposed  to  the  sky  sunk  below 
a  thermometer  suspended  four  feet  from  the  ground  as  follows : 
On  cloudless  nights,  9.3  degrees. 

"   nights  half  cloudy,  7.3       " 

"       "       principally  cloudy,  6.8       " 
"       "       entirely  cloudy,       3.4      " 

161.  Radiation  from  different  Substances. — Thermometers  placed 
on  different  substances  exhibit  very  unequal  reduction  of  tem- 
perature on  the  same  night.     When  a  thermometer  placed  on 
grass  sinks  ten  degrees  below  one  suspended  four  feet  from  the 
ground,  a  thermometer  placed  on  raw  wool  will  sink  12°  or  15° ; 
a  thermometer  placed  on  copper  will  sink  8° ;  on  paper,  6°  ;  and 
on  brick,  only  3°  or  4°.     Tab.  XXXII.  shows  the  average  results 
found  for  a  great  variety  of  substances.     These  numbers  indicate 
the  comparative  radiating  power  of  different  substances  for  heat. 

162.  Increase  of  Temperature  with  Elevation.  —  By  suspending 
thermometers  at  different  elevations  above  the  earth  from  one  or 


PRECIPITATION  OF  THE  VAPOR  OF  THE  AIR.  91 

two  inches  up  to  200  feet,  it  is  found  that  the  loss  of  heat  by  noc- 
turnal radiation  is  quite  sensible  at  the  elevation  of  50  feet,  and 
does  not  entirely  disappear  at  the  height  of  150  feet.    During  the 
night,  therefore,  the  temperature  of  the  air  increases  as  we  rise 
above  the  earth's  surface.     In  England,  according  to  the  average 
of  observations  continued  throughout  the  year,  if  a  thermometer 
placed  on  grass  fully  exposed  to  the  sky  be  taken  as  the  zero,  a 
thermometer  one  inch  above  it  would  read  3°  higher ; 
"          six  inches          "          "          6°      " 
"          one  foot  "  "          7°      " 

"          twelve  feet         "  "          8°      " 

"          fifty  feet  "  "        10°      " 

"          one  hundred  and  fifty  feet    12°       " 
and  the  effect  is  appreciable  at  still  greater  elevations. 

163.  Origin  of  Dew. — When,  in  consequence  of  radiation,  ob- 
jects near  the  earth's  surface,  such  as  grass  and  leaves  of  vegeta- 
bles, become  cooled  below  the  dew-point  in  the  vicinity,  they  con- 
dense upon  themselves  a  portion  of  the  vapor  which  is  present 
in  the  atmosphere,  in  the  manner  explained  in  Art.  99,  and  this 
moisture  is  called  deio.     The  amount  of  dew  thus  deposited  is 
greatest  upon  those  substances  whose  temperature  is  the  lowest, 
being  proportional  to  the  amount  of  depression  of  their  tempera- 
ture below  that  of  the  dew-point.     Dew,  therefore,  does  not  fall 
from  the  sky  like  drops  of  rain,  as  was  formerly  supposed,  but 
the  vapor  of  the  air  is  condensed  by  coming  in  contact  with  the 
cold  surface  of  the  object  upon  which  the  dew  collects. 

In  some  parts  of  the  world,  nearly  all  the  moisture  which  the 
earth  ever  receives  comes  in  the  form  of  dew.  This  is  particu- 
larly true  of  some  parts  of  Egypt  and  Arabia. 

164.  Circumstances  favorable  to  Dew. — The  circumstances  most 
favorable  to  the  production  of  dew  are  mainly  those  which  are 
most  favorable  to  the  loss  of  heat  by  radiation.     These  are, 

1st.  A  cloudless  night  and  unobstructed  exposure  to  the  sky. 
The  deposition  of  dew  is  immediately  checked  by  clouds  which 
reflect  back  to  the  earth  the  heat  radiated  from  it.  The  same 
effect  is  produced  by  any  artificial  covering,  even  though  of  the 
thinnest  texture.  Hence,  also,  plants  placed  beneath  a  tree  or  near 
a  building  collect  much  less  dew  than  those  which  are  freely  ex- 
posed to  the  sky. 


92  METEOROLOGY. 

2d.  A  nearly  tranquil  atmosphere.  A  slight  breeze,  by  renew- 
ing the  air  which  has  deposited  its  excess  of  vapor,  renders  the 
dew  more  abundant;  but  a  fresh  breeze,  by  agitation  of  the  air, 
produces  a  mingling  of  the  air  at  different  elevations,  equalizing 
the  temperature  throughout,  so  that  the  air  at  the  earth's  surface 
can  not  become  much  colder  than  the  superincumbent  atmos- 
phere. Little  dew  is  therefore  deposited  on  windy  nights. 

3d.  A  moist  atmosphere.  When  the  atmosphere  is  most  humid, 
a  given  reduction  of  temperature  will  sooner  reach  the  dew-point, 
at  which  the  deposition  of  moisture  begins.  An  abundant  dew 
is  regarded  as  an  indication  of  approaching  rain,  because  it  proves 
that  the  air  contains  a  large  quantity  of  vapor. 

4th.  Good  radiators  and  bad  conductors  of  heat  are  required 
for  collecting  the  dew.  Different  substances,  having  the  same 
exposure,  do  not  collect  the  same  amount  of  dew.  Wool  radiates 
heat  freely,  and,  being  a  bad  conductor,  collects  a  large  amount  of 
dew ;  while  but  little  dew  is  deposited  on  polished  metals,  since 
they  are  good  conductors  of  heat,  and  must  be  reduced  through- 
out to  a  low  temperature  before  any  dew  can  be  deposited  upon 
them.  If  similar  plates  of  polished  glass  and  steel  are  exposed 
alike  upon  the  ground  during  a  favorable  night,  in  the  morning 
the  glass  will  be  drenched  with  dew,  while  the  brightness  of  the 
metal  will  be  scarcely  dimmed.  The  glass  radiates  heat  more 
rapidly  than  the  metal,  and,  being  a  bad  conductor,  draws  but  little 
warmth  from  the  earth  to  supply  its  loss;  while  the  metal,  being 
a  good  conductor,  readily  derives  heat  from  the  warm  soil  below. 

165.  Dew  during  the  Day. — The  deposition  of  dew  sometimes 
commences  before  sunset.     It  continues  at  all  hours  of  the  night, 
provided  the  weather  remains  favorable ;  but  more  dew  is  formed 
after  midnight  than  before ;  and  the  deposition  sometimes  con- 
tinues after  sunrise. 

In  places  sheltered  from  the  sun,  but  open  to  a  considerable 
portion  of  the  sky,  dew  is  sometimes  deposited  on  grass  even  at 
midday. 

166.  Where  there  is  no  Dew. — Dew  is  not  deposited  on  the  sur- 
face of  large  bodies  of  water  whose  temperature  is  above  40°,  for 
as  soon  as  the  particles  at  the  surface  are  cooled  they  become 
heavier  and  sink,  while  warmer  and  lighter  particles  rise  to  the 


PRECIPITATION   OF  THE   VAPOR  OF   THE   AIR.  93 

top,  by  which  means  the  surface  of  the  water  is  maintained  at 
nearly  the  same  temperature  as  the  surrounding  air. 

In  the  midst  of  sandy  deserts,  on  account  of  the  dryness  of  the 
atmosphere,  dew  is  almost  entirely  unknown.  Travelers  upon 
the  deserts  of  Africa  and  Asia  are  notified  of  their  proximity  to 
lakes  or  rivers  by  the  appearance  of  dew. 

But  little  dew  is  deposited  in  cities,  because  most  of  the  objects 
there  found  are  poorer  radiators  than  the  leaves  of  vegetables, 
and  because  the  heat  of  the  city  is  always  greater  than  that  of  the 
surrounding  country. 

167.  Amount  of  Dew  determined. — Attempts  have  been  made  to 
determine  the  total  amount  of  dew  annually  deposited  in  differ- 
ent countries.     This  is  sometimes  done  by  exposing  a  plate  of 
glass  or  some  other  substance  to  the  sky,  and  carefully  weighing 
the  amount  of  moisture  deposited  upon  it.     In  this  way  it  has 
been  concluded  that  in  Italy  and  the  south  of  France  the  annual 
deposit  of  dew  amounts  to  a  little  more  than  a  quarter  of  an  inch. 
Such  results,  however,  are  not  very  reliable,  since  they  are  great- 
ly influenced  by  the  radiating  power  of  the  plate  employed,  and 
also  by  its  position. 

SECTION  II. 

HOAR-FROST. 

168.  Formation  of  Hoar-frost. — Hoar-frost  is  formed  under  the 
same  circumstances  as  dew,  with  the  exception  of  a  lower  tem- 
perature.    When  the  temperature  of  plants  falls  below  32°  the 
moisture  of  the  air  is  condensed  upon  them  in  the  solid  state,  and 
forms  a  layer  of  spongy  ice.     Hoar-frost,  therefore,  is  not  frozen 
dew,  but  the  moisture  of  the  air  is  deposited  in  the  solid  form,  with- 
out having  passed  through  the  liquid  condition.     Hoar-frost,  like 
dew,  is  deposited  chiefly  upon  those  bodies  which  radiate  best, 
such  as  plants  and  the  leaves  of  vegetables,  and  the  deposit  is 
made  principally  on  those  parts  which  are  turned  toward  the  sky. 

Since  plants  sometimes  become  cooled  by  radiation  from  12° 
to  15°  below  the  temperature  of  the  surrounding  air,  a  frost  may 
occur  although  a  thermometer  a  few  feet  above  the  ground  does 
not  sink  to  32°.  During  a  clear  and  still  night,  when  a  thermom- 
eter six  feet  above  the  ground  sinks  to  36°,  a  very  heavy  frost 


94  METEOROLOGY. 

may  be  expected,  and  a  slight  frost  may  occur  when  the  same 
thermometer  sinks  only  to  47°. 

169.  How  Plants  are  protected  from  Frost. — Whatever  prevents 
the  radiation  of  heat  serves  also  to  check  the  formation  of  hoar- 
frost.    During  the  cold  nights  of  spring,  plants  which  are  shel- 
tered by  trees  are  less  liable  to  be  injured  by  frost  than  those 
which  are  fully  exposed,  and  a  thin  covering  of  cloth  or  straw 
will  generally  afford  entire  protection.     A  garden  may  frequent- 
ly be  saved  from  injury  by  kindling  a  small  fire,  which  shall  en- 
velop the  plants  in  a  cloud  of  smoke.     Fogs  and  clouds  also  pro 
tect  vegetation  from  the  effects  of  frost. 

170.  Frost  in  Valleys. — Plants  are  often  killed  by  frost  in  the  val- 
leys and  up  to  a  certain  height  upon  the  hills,  while  above  this 
limit  they  entirely  escape  injury.     It  has  been  found  by  observ- 
ation that  a  thermometer  attached  to  a  high  tower  in  a  valley  in- 
dicates at  night  the  same  average  temperature  as  a  thermometer 
on  the  side  of  a  neighboring  hill  upon  the  same  level.     This  in- 
dicates that  during  a  tranquil  night  the  cold  air  resulting  from 
radiation  at  the  surface  of  the  earth  settles  in  the  valleys  in  con- 
sequence of  its  greater  density,  and  the  warm  and  cold  air  are  ar- 
ranged in  nearly  horizontal  strata  like  liquids  of  different  densities. 

171.  Crystalline  Structure  of  Hoar-frost. — Hoar-frost  generally  ex- 
hibits a  crystalline  structure,  and  consists  of  long  spicula3,  which 
are  found  to  be  hexagonal  prisms  with  angles  of  120°.      These 
spicula3  are  frequently  seen  in  great  perfection  in  the  frost  which 
forms  on  wooden  fences,  on  the  decayed  branches  of  trees,  etc. 

When  a  thin  film  of 
water  freezes  upon  a 
flat  surface  of  glass  or 
stone,  it  often  forms  a 
great  variety  of  beau- 
tiful  figures,  some- 
times resembling  the 
leaves  of  certain 
plants,  the  leaves  of 
the  palm-tree,  or  the 
feathers  of  birds,  Figs. 


Fig. 44 


PRECIPITATION   OF  THE   VAPOR   OF  THE   AIR.  95 

43  and  44.  In  cold  weather,  smooth  and  flat  stones  upon  the 
side-walk  are  often  covered  with  these  figures,  which,  upon  ex- 
amination, are  found  to  consist  mainly  of  spiculae  more  or  less 
perfectly  formed. 

A  species  of  hoar-frost  occurs  when  a  warm  wind  succeeds  a 
period  of  severe  cold  weather.  Stone  buildings  are  then  often 
covered  with  an  incrustation  of  minute  crystals  caused  by  the 
low  temperature  of  the  stone,  which  condenses  and  congeals  the 
moisture  of  the  air. 

SECTION  III. 

FOG. 

172.  Condensation  of  the  Vapor  of  the  Atmosphere.  —  The  vapor 
in  the  atmosphere  is  nearly  or  quite  transparent;  but  when,  from 
any  cause,  the  air  becomes  cooled  below  the  dew-point,  a  portion 
of  its  vapor  is  precipitated  in  the  form  of  drops  of  water  ex- 
tremely minute,  which  affect  the  transparency  of  the   air,  and 
form  fog  or  cloud  according  as  it  occurs  near  the  surface  of  the 
earth,  or  in  the  upper  regions  of  the  atmosphere.     If  we  com- 
press moist  air  in  a  close  vessel  and  allow  it  suddenly  to  escape, 
the  air,  by  its  expansion,  will  be  cooled,  and  a  slight  fog  be  pro- 
duced ;  but  the  air  soon  regains  its  warmth,  the  drops  of  water 
return  to  the  state  of  vapor,  and  the  fog  is  dissipated. 

When  steam  rises  from  a  vessel  of  warm  water  and  mingles 
with  a  cold  atmosphere,  a  portion  of  the  vapor  is  condensed  and 
a  mist  is  formed.  This  mist  is  sometimes,  but  improperly,  called 
vapor.  Vapor  of  water  is  a  gaseous  body,  while  mist  is  a  liquid 
body.  A  similar  condensation  often  takes  place  in  nature  upon 
a  large  scale,  and  the  mist  is  then  called  a  fog. 

173.  Fogs  over  Rivers  in  Summer. — At  certain  seasons  of  the 
year,  especially  during  the  latter  part  of  summer,  upon  nearly 
every  clear  and  still  night,  fogs  form  over  rivers  and  lakes.     At 
night  the  temperature  of  the  air  over  the  land  becomes  cooler 
than  the  water  of  lakes  and  rivers.     The  vapor  which  rises  at 
such  a  time  from  the  warm  water  is  condensed  by  contact  with 
the  cooler  air  from  the  land,  and  a  fog  is  formed,  which  seems  to 
rest  upon  the  water. 

On  a  clear  and  quiet  morning  in  the  month  of  August,  an  ob- 
server on  the  summit  of  Mount  Washington  sees  the  bed  of  the 


96  METEOROLOGY. 

Connecticut  Eiver  distinctly  traced  by  a  long  line  of  fog,  and  the 
position  of  a  multitude  of  surrounding  lakes  is  indicated  in  the 
same  manner,  while  other  portions  of  the  country  are  entirely 
free  from  fog. 

That  this  fog  is  formed  by  the  vapor  of  the  warm  water  rising 
into  an  atmosphere  which  is  cooler  than  the  water,  is  proved  by 
observations  of  the  thermometer.  On  a  morning  in  July,  when 
the  Connecticut  River  was  covered  with  a  thick  fog,  the  tempera- 
ture of  the  water  was  found  to  be  73°,  while  the  temperature  of 
the  air  over  the  neighboring  land  was  only  68°. 

Such  fogs  generally  disappear  soon  after  sunrise.  Sometimes, 
from  the  effect  of  the  sun's  heat,  they  are  seen  to  ascend  and  rise 
above  the  hills,  forming  clouds,  which  soon  disappear  with  the 
increasing  heat  of  the  sun. 

Fogs  are  often  formed  in  a  similar  manner  over  harbors,  bays, 
etc.,  and  these  fogs,  by  a  gentle  current,  are  often  drifted  over  the 
land.  In  this  manner  a  sea  fog  sometimes  spreads  over  the  city 
of  New  York,  and  extends  several  miles  up  the  Hudson  River. 

174.  Fogs  in  Spring  and  Winter.  —  During  the  spring  of  the 
year,  fogs  are  sometimes  formed  over  rivers,  when  the  tempera- 
ture of  the  water  is  colder  than  that  of  the  surrounding  air.     In 
this  case  the  warm  and  moist  air  of  the  neighboring  land  is  chill- 
ed by  coming  in  contact  with  the  cold  water,  and  a  portion  of  its 
vapor  is  condensed. 

In  the  same  manner,  after  a  warm  rain  in  mid- winter,  dense 
fogs  are  sometimes  formed  by  a  warm  and  moist  air  flowing  over 
a  country  which  is  covered  with  snow ;  or  the  fog  may  result 
from  the  moist  air  becoming  cooled  by  contact  with  a  frozen  soil. 
Indeed,  a  fog  may  be  formed  at  any  time  at  a  distance  from  large 
bodies  of  water,  when  the  vapor  which  rises  from  a  very  moist 
soil  mixes  with  a  cold  atmosphere. 

In  the  same  manner,  fogs  are  often  formed  on  the  sides  of 
mountains.  The  warm  air  from  the  valleys  being  forced  up  the 
sides  of  the  mountain,  its  vapor  is  condensed,  partly  by  the  cold 
of  elevation,  and  partly  by  contact  with  the  cold  surface  of  the 
mountain. 

175.  Where  Fogs  are  most  Prevalent. — On  the  Atlantic  Ocean, 
Jrom  30°  south  to  35°  north  latitude,  fogs  are  almost  unknown. 


PRECIPITATION   OF  THE  VAPOE  OF  THE  AIB.  97 

On  the  northern  side  of  the  Gulf  Stream  they  are  of  common 
occurrence,  but  they  are  most  prevalent  near  the  Banks  of  New- 
foundland. These  fogs  occur  in  every  month  of  the  year,  but  they 
are  most  prevalent  in  summer,  when  the  Banks  are  enveloped 
in  fog  nearly  half  the  time.  The  vapor  which  causes  these  fogs 
is  furnished  by  the  warm  air  of  the  Gulf  Stream,  and  it  is  con- 
densed by  the  cold  air  of  the  Banks,  the  contrast  of  temperature 
being  here  more  sudden  than  is  found  in  any  other  part  of  the 
Atlantic  Ocean.  During  the  month  of  July  the  water  on  the 
Banks  frequently  has  a  temperature  of  45°,  while  within  a  dis- 
tance of  less  than  300  miles  the  Gulf  Stream  has  a  temperature 
of  78°.  The  contrast  of  temperature  is  almost  equally  great  in 
January,  but  fogs  are  less  frequent  in  winter,  because  at  that  pe- 
riod the  air  is  more  agitated  by  storms,  which  tend  to  equalize  the 
temperature  over  different  parts  of  the  ocean. 

In  the  South  Atlantic,  beyond  the  parallel  of  30°,  fogs  are  of 
common  occurrence,  but  they  are  nowhere  so  prevalent  as  on  the 
Banks  of  Newfoundland. 

176.  Fogs  of  Polar  Regions,  etc. — Fogs  are  very  prevalent  in  the 
Arctic  regions,  particularly  in  summer.     During  an  Arctic  sum- 
mer the  temperature  of  the  earth  rises  much  above  that  of  the 
sea,  portions  of  which  are  covered  with  immense  fields  of  ice. 
The  air  resting  upon  the  earth  partakes  of  its  temperature,  and 
when  this  warmer  air  is  brought  in  contact  with  the  colder  ocean, 
a  portion  of  its  vapor  is  condensed,  and  a  heavy  mist  is  formed. 

During  winter,  England  and  the  neighboring  portions  of  the 
Continent  are  frequently  enveloped  in  dense  fogs,  and  in  those 
towns  where  bituminous  coal  is  used  abundantly  the  sky  is  some- 
times so  darkened  by  the  mixture  of  fog  and  smoke  that  locomo- 
tion even  at  midday  becomes  almost  impossible.  In  London, 
during  winter,  the  streets  are  sometimes  lighted  with  gas  all  day, 
and  travel  through  the  city  is  attended  with  serious  danger. 
This  fog  results  from  the  warm  air  of  the  sea  spreading  over  the 
cold  land. 

177.  Where  Fogs  do  not  Prevail. — Fogs  are  never  formed  when 
the  air  is  very  dry,  and  therefore  they  are  never  known  in  deserts. 

Fogs  are  not  common  in  tropical  countries  except  in  the  neigh- 

G 


98  METEOROLOGY. 

borhoocl  of  mountains  ;  but  the  summits  of  mountains,  even  un- 
der the  equator,  are  habitually  shrouded  in  fog  or  cloud. 

178.  The  Vesicular  Theory. — Since  fog  consists  of  particles  of  a 
liquid  which  is  nearly  eight  hundred  times  denser  than  the  air, 
it  has  been  thought  difficult  to  explain  how  it  can  be  sustained  in 
the  atmosphere.     Some  have  supposed  that  the  particles  of  fog 
are  hollow,  each  consisting  of  a  sphere  of  air  surrounded  by  a 
thin  envelope  of  water  like  a  soap-bubble.    Such  a  hollow  sphere 
is  called  a  vesicle,  and  this  theory  of  the  constitution  of  fog  is  call- 
ed the  vesicular  theory. 

179.  Argument  from  the  appearance  of  Mist.  —  Some  observers 
who  have  examined  with  a  magnifying-glass  the  particles  of  mist 
vising  from  hot  water,  have  detected  on  their  surface  colored  rings 
like  those  seen  on  soap-bubbles,  indicating  that  their  structure 
was  analogous  to  that  of  soap  -  bubbles,  and  it  has  been  inferred 
that  the  particles  of  fog  generally  have  a  similar  constitution. 
Water  ordinarily  contains  minute  bubbles  of  air,  and  when  the 
water  is  warmed  these  bubbles  expand  and  rise  to  the  surface. 
They  often  rest  upon  the  surface  of  warm  water,  and,  being  sur- 
rounded by  a  thin  film  of  water,  they  should  exhibit  colored  rings 
like  soap-bubbles,  but  there  is  no  evidence  that  the  particles  of 
fog  are  generally  so  constituted.    A  fog  is  formed  from  the  vapor 
of  water  previously  existing  in  the  air  in  the  gaseous  state,  and 
when  this  vapor  returns  to  the  liquid  condition  there  is  no  evi- 
dence that  it  assumes  the  form  of  hollow  spheres. 

180.  Argument  from  the  absence  of  a  Rainbow. — It  is  contended 
that  the  particles  of  fog  can  not  be  solid  spheres  of  water,  because 
if  they  were,  a  rainbow  should  be  seen  whenever  the  spectator  is 
situated  between  the  sun  and  a  fog.     A  rainbow  is  formed  by  the 
reflection  of  the  sun's  rays  from  falling  drops  of  water.     These 
drops  are  spheres  of  water;  and  since  a  fog  does  not  form  a  rain- 
bow, it  is  contended  that  the  particles  of  the  fog  can  not  be  solid 
drops.     But  it  has  been  shown  that  when  the  spheres  of  water 
are  very  small,  as  is  the  case  with  a  fog,  a  bow  should  indeed  be 
formed,  but  the  different  colored  bands  are  very  broad,  and  their 
light  is  proportionally  feeble.     Moreover,  if  the  spheres  are  not 
all  sensibly  of  the  same  diameter,  there  will  be  formed  simultane- 


PRECIPITATION   OF   THE   VAPOR   OF   THE   AIR.  99 

ously  bands  of  different  breadths,  which  will  be  superposed  upon 
each  other  in  such  a  manner  that  the  different  colors  will  be  very 
much  blended,  and  produce  a  light  which  is  nearly  white,  form- 
ing thus  a  very  faint  and  nearly  white  rainbow,  or  rather  fog-bow, 
which  corresponds  exactly  with  the  facts.  When  a  spectator  is 
situated  between  the  sun  and  a  bank  of  fog,  a  white  bow  is  often 
seen  with  but  little  appearance  of  prismatic  colors,  and  the  breadth 
of  the  bow  is  about  double  that  of  an  ordinary  rainbow. 

Thus  the  absence  of  the  common  rainbow  in  fogs  not  only 
does  not  establish  the  vesicular  theory,  but  the  existence  of  the 
white  fog-bow  positively  refutes  this  theory. 

181.  Argument  from  the  Constitution  of  Clouds. — Fogs  evidently 
have  the  same  constitution  as  clouds.     Now,  when  clouds  are 
formed  at  a  low  temperature,  their  particles  are  solid,  consisting 
of  spiculaB  of  ice,  which,  united,  form  flakes  of  snow.    But  we  find 
nothing  of  the  vesicular  constitution  in  snow-flakes;  nevertheless, 
clouds  composed  of  spiculse  of  ice  remain  suspended  in  the  air  for 
hours,  and  sometimes  days  in  succession.    The  vesicular  hypothe- 
sis, therefore,  is  not  necessary  to  account  for  the  permanence  of 
clouds  and  fogs,  and  there  is  no  evidence  that  they  are  ever  thus 
constituted. 

182.  How  Fog  is  sustained  in  the  Air. — The  particles  of  fog  are 
sustained  in  the  air  in  the  same  manner  as  a  cloud  of  dust  is 
sustained.     A  cloud  of  dust  remains  for  a  long  time  suspended 
in  the  air,  although  each  particle  of  dust  may  consist  of  matter 
two  thousand  times  as  dense  as  the  air  in  which  it  floats.    When 
the  air  is  perfectly  tranquil  these  particles  do  indeed  fall,  but  they 
descend  so  slowly  that  their  motion  is  only  perceptible  after  the 
lapse  of  a  considerable  interval  of  time. 

183.  Diameter  of  Particles  of  Fog. — The  diameter  of  particles 
of  fog  is  very  variable,  being  sometimes  so  small  that  the  indi- 
vidual particles  can  not  be  separately  seen,  and  it  is  only  in  mass 
that  they  make  any  impression  upon  the  eye,  and  they  are  found 
increasing  in  size  until  they  fall  with  considerable  velocity,  when 
they  are  called  rain-drops. 

The  diameter  of  the  smallest  visible  particles  of  fog  has  been 
estimated  at  -j-g-jr  inch ;  and  when  the  diameter  of  the  particles  be- 


100  METEOROLOGY. 

comes  equal  to  ¥V  inch,  they  fall  with  an  appreciable  velocity,  and 
are  called  rain-drops. 

184.  Indian  Summer. — At  certain  seasons  of  the  year  there 
occurs  a  peculiar  phenomenon  called  a  dry  fog.     In  the  United 
States  this  frequently  occurs  in  November,  or  the  latter  part  of 
October,  and  this  period  is  commonly  known  by  the  name  of  In- 
dian Summer.     This  period  is  characterized  by  a  hazy  condition 
of  the  atmosphere,  a  redness  of  the  sky,  absence  of  rain,  and  a 
mild  temperature.     This  appears  to  result  from  a  dry  and  stag- 
nant state  of  the  atmosphere,  during  which  the  air  becomes  filled 
with  dust  and  smoke  arising  from  numerous  fires,  by  which  its 
transparency  is  greatly  impaired.    A  heavy  rain  washes  out  these 
impurities  and  effectually  clears  the  sky. 

This  phenomenon  is  not  peculiar  to  the  United  States,  a  simi- 
lar condition  of  the  atmosphere  being  frequently  observed  in 
Central  Europe.  Moreover,  this  dry  and  stagnant  state  of  the 
atmosphere  is  not  limited  to  a  single  season  of  the  year.  The 
long  periods  of  drought  which  frequently  prevail  in  summer  are 
characterized  by  a  like  condition  of  the  atmosphere. 

185.  Prevalence  of  Volcanic  Ashes,  etc.  —  Sometimes  a  dry  fog 
continues  for  several  weeks,  and  prevails  over  a  vast  area,  ex- 
hibiting very  peculiar  characteristics.     These  fogs  have  been  as- 
cribed to  the  presence  of  fine  volcanic  ashes  in  the  atmosphere, 
and  perhaps  also  of  substances  foreign  to  the  earth. 

In  1783  such  a  fog  prevailed  over  all  Europe,  and  continued 
for  more  than  a  month.  It  was  preceded  by  a  remarkable  erup- 
tion of  the  volcano  Hecla,  in  Iceland,  which  for  a  long  time  emitted 
smoke  of  unusual  density. 

In  1831  a  similar  fog  prevailed  in  the  United  States,  in  Europe, 
and  on  the  coast  of  Africa.  It  obscured  the  air  to  such  an  extent 
that  the  sun  could  be  observed  all  day  with  the  naked  eye,  with- 
out the  interposition  of  any  colored  glass.  At  night  the  fog 
seemed  decidedly  phosphorescent,  and  emitted  an  appreciable 
amount  of  light,  which  could  not  be  ascribed  to  the  reflected  light 
of  the  stars. 


PRECIPITATION   OF   THE   VAPOR   OF   THE   AIR.  101 

SECTION  IV. 

CLOUDS. 

186.  Clouds  differ  from  fogs  only  in  their  elevation  above  the 
earth.     A  fog  resting  on  the  top  of  a  mountain  is  called  a  cloud. 
A  cloud  resting  on  the  surface  of  the  earth  is  called  a  fog. 

187.  Classification  of  Clouds — Cirrus.  —Clouds  present  an  in- 
finite variety  of  forms,  yet  they  ma}'  be  divided  into  six  classes, 
each  presenting  characteristics  tolerably  distinct.     Three  of  these 
modifications  are  primary,  and  three  are  compound. 

The  cirrus  cloud  consists  of  long,  slender  filaments,  either  par- 
allel or  diverging  from  each  other,  and  often  presents  the  appear- 
ance of  a  lock  of  cotton  whose  fibres  are  electrified  so  as  power- 
fully to  repel  each  other.  These  clouds  appear  to  have  the  least 
density,  the  greatest  elevation,  and  the  greatest  variety  of  form. 
They  are  generally  the  first  to  make  their  appearance  after  a  pe- 
riod of  perfectly  clear  weather.  Indeed,  in  fair  weather,  the  sky 
is  seldom  entirely  free  from  small  groups  of  cirrus  clouds.  They 
are  believed  to  be  composed  of  spiculae  of  ice  or  flakes  of  snow, 
floating  at  a  great  height  in  the  air.  At  the  height  at  which  they 
prevail,  the  temperature  of  the  air  is  below  32°  even  in  midsum- 
mer. It  is  among  clouds  of  this  variety  that  halos  and  parhelia 
are  formed,  phenomena  which  are  ascribed  to  the  refraction  of 
light  by  minute  prisms  of  ice. 

188.  Cumulus. — The  cumulus  cloud  usually  consists  of  a  hemi- 
spherical or  convex  mass,  rising  from  a  horizontal  base.     It  is 
much  denser  than  the  cirrus,  and  is  formed  in  the  lower  regions 
of  the  atmosphere.     In  fair  weather  the  cumulus  often  forms  a 
few  hours  after  sunrise,  goes  on  increasing  until  the  hottest  part 
of  the  day,  and  disappears  about  sunset.     We  often  see  near  the 
horizon  large  masses  of  cumulus  clouds,  which  resemble  lofty 
mountains  covered  with  snow. 

The  rounded  top  of  the  cumulus  results  from  the  mode  of  its 
formation.  When  the  surface  of  the  earth  is  heated  by  the  rays 
of  the  sun,  currents  of  warm  air  ascend,  and  as  soon  as  they  reach 
a  certain  height  a  portion  of  their  vapor  is  condensed,  and  forms 
cloud ;  and  since  the  upward  motion  is  greatest  under  the  centre 
of  the  cloud,  the  vapor  is  there  carried  up  to  the  greatest  height 


102  METEOROLOGY. 

In  like  manner,  when  steam  escapes  in  large  quantities  from  the 
boiler  of  a  steam-engine,  especially  in  a  damp  atmosphere,  it  forms 
a  rounded  mass  of  mist. 

189.  Stratus. — The  stratus  cloud  is  a  widely-extended,  continu- 
ous, horizontal  sheet,  often  covering  the  entire  sky  with  a  nearly 
uniform  veil.     This  is  the  lowest  of  the  clouds,  and  sometimes 
descends  to  the  earth's  surface. 

190.  Compound  Modifications. — The  cirro-cumulus  consists  of 
small,  well -defined,  rounded  masses,  in  close  proximity.     These 
little  rounded  clouds,  on  account  of  their  fleecy  appearance,  are 
sometimes  called  woolly  clouds.     The  cirro-cumulus  is  frequent 
in  summer,  and  is  attendant  on  warm  and  dry  weather. 

The  cirro-stratus  consists  of  delicate  fibrous  clouds  spread  out 
in  strata,  which  are  either  horizontal,  or  but  slightly  inclined  to 
the  horizon.  This  cloud  appears  to  result  from  the  subsidence 
of  the  fibres  of  the  cirrus  to  a  horizontal  position.  Sometimes 
the  whole  sky  is  so  mottled  with  this  kind  of  cloud  as  to  resem- 
ble the  back  of  a  mackerel,  and  is  hence  called  the  mackerel-sky. 
The  cirro-stratus  precedes  wind  and  rain,  and  is  almost  always  to 
be  seen  in  the  intervals  of  storms. 

The  cumulo •  stratus  consists  of  the  cumulus  blended  with  the 
stratus,  and  is  formed  in  the  interval  between  the  first  appear- 
ance of  the  fleecy  cumulus  and  the  commencement  of  rain.  On 
the  approach  of  a  thunder-storm  the  cumulo-stratus  clouds  are 
often  seen  in  great  magnificence,  and  present  those  peculiar  forms 
known  in  some  places  by  the  name  of  thunder-heads.  All  these 
varieties  of  cloud  are  represented  in  Plate  II. 

191.  Best  Mode  of  observing  the  Clouds. — In  order  to  be  able  to 
distinguish  well  the  form  of  clouds,  it  is  often  necessary  to  di- 
minish their  brilliancy  by  viewing  them  through  a  glass  of  a 
deep  blue  color,  or  by  reflection  from  a  mirror  of  black  glass. 
We  are  thus  able  to  detect  peculiarities  which  entirely  escape 
observations  with  the  unassisted  eye. 

The  appearance  of  a  cloud  often  changes  greatly  with  its 
change  of  position  in  the  heavens.  The  peculiarities  of  clouds 
are  generally  more  noticeable  when  they  are  near  the  zenith 
than  when  they  are  near  the  horizon. 


PRECIPITATION   OF  THE   VAPOR   OF   THE   AIR.  103 

Besides  the  six  modifications  of  clouds  above  enumerated, 
Howard  admitted  a  seventh,  which  he  called  the  cumulo-cirro- 
stratus,  or  nimbus,  to  denote  a  cloud  or  system  of  clouds  from 
which  rain  is  falling;  but  it  is  often  so  difficult  to  distinguish  be- 
tween the  stratus  and  nimbus  that  it  seems  inexpedient  to  retain 
the  last  division. 

192.  Average  degree  of  Cloudiness. — Clouds  are  more  prevalent 
in  some  parts  of  the  world  than  in  others.     Throughout  New 
England,  on  an  average  for  the  whole  year,  T^j-ths  of  the  sky  are 
covered  with  clouds,  while  in  the  Southern  States  the  average 
is  -j^ths.     Near  the  equator,  between  the  N.  E.  and  S.  E.  trade 
winds,  there  are  places  where  the  sky  is  almost  constantly  cov- 
ered with  clouds.     At  St.  Helena,  at  an  elevation  of  1764  feet,  on 
an  average  for  the  whole  year,  -^nrths  of  the  sky  are  covered  with 
clouds,  and  on  the  tops  of  high  mountains  the  sky  is  seldom  free 
from  clouds. 

Throughout  most  of  Great  Britain  the  average  cloudiness  is 
-j^ths,  while  at  Bombay  it  is  only  -j^ths,  and  at  Sacramento, 
California,  it  is  only  f^ths. 

193.  Height  of  Clouds. — The  height  of  a  cloud  may  sometimes 
be  measured  in  the  same  manner  as  the  height  of  any  other  in- 
accessible object,  by  simultaneous  observations  of  its  direction  at 
two  stations.     More  satisfactory  results  may,  however,  be  obtain- 
ed by  ascending  in  a  balloon,  and  noting  the  height  of  the  ba- 
rometer at  the  instant  of  entering  a  cloud,  and  again  when  emerg- 
ing from  it ;   the  barometer  affording  the  means  of  computing 
the  corresponding  altitudes.     In  mountainous  countries  we  may 
sometimes  determine  the  height  of  a  cloud  by  comparing  it  with 
some  peak  of  known  elevation  near  which  the  cloud  is  carried 
by  the  wind. 

The  height  of  clouds  is  very  variable,  and  their  mean  eleva- 
tion is  not  the  same  in  different  countries.  The  stratus  cloud 
often  descends  to  the  earth's  surface.  In  pleasant  weather,  the 
lower  limit  of  cumulus  clouds  varies  from  3000  to  5000  feet  ele- 
vation, and  their  upper  limit  from  5000  to  12,000  feet.  Cirrus 
clouds  are  never  seen  below  the  summit  of  Mount  Blanc,  which 
has  an  elevation  of  15,744  feet. 

Clouds  are  sometimes  seen  above  the  summit  of  Chimborazo, 


104  METEOROLOGY. 

which  has  an  elevation  of  21,424  feet.  Gay-Lussac  and  Glaisher, 
in  their  different  balloon  ascents  to  the  height  of  23,000  feet,  saw 
cirrus  clouds  which  appeared  considerably  above  them.  It  is 
estimated  that  the  greatest  height  at  which  visible  clouds  ever 
exist  does  not  exceed  ten  miles. 

194.  Vertical  Thickness  of  Clouds.  —  The  vertical  thickness  of 
clouds  does  not  generally  exceed  half  a  mile,  but  cumulus  clouds 
are  sometimes  formed  of  enormous  magnitude  and  height.     It 
has  been  computed  that  the  tops  of  cumulus  clouds  sometimes 
attain  the  height  of  four  miles,  while  their  bases  are  not  more 
than  half  a  mile  above  the  earth's  surface. 

195.  Formation  of  Clouds. — The  vapor  generated  at  the  surface 
of  the  earth  by  the  heat  of  the  sun  tends,  by  its  expansive  force 
to  spread  in  all  directions,  but  especially  upward,  and  forms  an 
atmosphere  of  vapor  whose  density  decreases  with  the  elevation. 
Since  the  temperature  of  the  air  sinks  as  we  rise  above  the  earth, 
it  may  happen  that  at  a  certain  height  the  tension  of  the  vapor 
is  greater  than  corresponds  to  the  temperature  which  prevails  at 
that  elevation,  in  which  case  a  portion  of  the  vapor  will  be  pre- 
cipitated and  form  a  cloud. 

Vapor  may  also  be  transported  to  a  great  height  by  the  ascend- 
ing currents  of  air  caused  by  the  heat  of  the  sun.  These  currents 
ordinarily  give  rise  to  cumulus  clouds,  and  it  frequently  happens 
that  the  sky,  though  clear  in  the  morning,  is  filled  with  those 
clouds  at  noon. 

Any  cause  which  chills  a  humid  air  determines  the  formation 
of  cloud.  A  cold  wind  penetrating  a  humid  air,  or  a  warm  and 
humid  wind  penetrating  a  cold  air,  causes  the  precipitation  of 
vapor  and  the  formation  of  clond.  At  the  close  of  a  warm  day, 
especially  after  a  rain,  clouds  are  frequently  formed,  which  in- 
crease during  the  night,  and  are  dissipated  the  next  day  by  the 
effect  of  the  sun's  heat. 

196.  Summits  of  Mountains  enveloped  in  Cloud. — The  summits 
of  high  mountains  are  almost  always  enveloped  in  clouds,  even 
though  every  other  portion  of  the  sky  is  perfectly  clear.     This  is 
not  due  to  any  attraction  between  the  mountain  and  the  cloud, 
but  rather  the  mountain  causes  the  cloud.     The  effect  cf  an  inter- 


PRECIPITATION   OF  THE   VAPOR   OF   THE   AIR. 


105 


posed  mountain  is  to  force  a  horizontal  wind  up  to  an  unusual 
height  where  the  temperature  is  low,  and  when  the  temperature 
of  the  air  is  reduced  below  its  dew-point,  a  portion  of  its  vapor 

must  be  condensed  and 
form  cloud.  Thus,  let 
ABC  be  a  mountain 
interposed  in  the  path 
of  a  horizontal  current 
of  air.  The  air  is  by 
this  means  forced  up- 
ward, and  made  to  glide 
along  the  side  of  the 
mountain.  If  DE  rep- 
resents the  height  at  which  the  temperature  of  the  air  is  just  equal 
to  the  dew-point  of  this  current,  then,  as  soon  as  the  wind  passes 
above  the  line  DE,  a  portion  of  its  vapor  will  be  condensed,  and 
a  cloud  will  be  formed  which  will  envelop  the  summit  of  the 
mountain.  But  when  the  air,  descending  from  the  mountain  on 
the  opposite  side,  passes  below  the  line  DE,  it  attains  a  tempera- 
ture which  is  above  the  dew-point,  and  the  cloud  is  redissolved. 

It  is  sometimes  thought  strange  that  the  strong  wind  which 
usually  prevails  on  the  summits  of  mountains  does  not  Now  away 
the  cloud.  Undoubtedly  the  cloud  is  drifted  off  with  the  wind,  but 
its  place  is  instantly  supplied  with  new  cloud.  Thus,  although 
the  cloud  on  the  summit  of  the  mountain  appears  perfectly  sta- 
tionary, the  particles  which  compose  the  cloud  are  continually 
changing.  A  somewhat  similar  effect  often  takes  place  over 
countries  which  are  tolerably  level.  The  sky  does  not  become 
overcast  solely  from  clouds  which  are  drifted  by  the  wind  from 
places  beyond  the  horizon ;  but  new  clouds  frequently  form  di- 
rectly in  sight  of  an  observer.  On  the  contrary,  a  cloudy  sky 
sometimes  clears  up,  not  because  the  clouds  are  drifted  off  by  the 
wind,  but  because  they  are  converted  into  vapor  by  the  increas- 
ing heat  of  the  air. 

197.  How  Clouds  are  Sustained. — Since  clouds  consist  of  parti- 
cles which  are  heavier  than  the  surrounding  air,  they  "must  sink, 
even  though  it  be  slowly,  and  we  might  conclude  that  in  calm 
weather  they  must  at  length  fall  to  the  ground.  The  particles  of 
a  cloud,  however,  in  pleasant  weather  can  not  reach  the  ground, 


106  METEOROLOGY. 

because  in  descending  they  meet  a  warmer  stratum  of  air  which 
is  not  saturated  with  vapor,  when  the  lower  part  is  again  con- 
verted into  vapor  and  disappears.  This  explains  why  the  base 
of  the  cumulus  cloud  is  uniformly  horizontal.  While,  however, 
the  particles  on  the  lower  side  of  the  cloud  are  dissolved,  the  up- 
per part" of  the  cloud  is  continually  increasing  by  the  condensa- 
tion of  new  vapor,  which  is  carried  upward  by  ascending  currents 
of  air,  by  which  means  the  cloud  appears  to  maintain  a  constant 
elevation  above  the  earth. 

198.  Currents  in  the  Air. — Observations  of  the  clouds  often  dis- 
close  the  existence  of  currents  in  the  atmosphere  flowing  in  vari- 
ous and  perhaps  opposite  directions.     We  sometimes  notice  a 
stratum  of  clouds  moving  nearly  in  the  direction  of  the  air  at  the 
earth's  surface,  while  at  a  greater  elevation  we  observe  a  stratum 
moving  in  a  different  direction,  and  sometimes  a  third  and  per- 
haps a  fourth  moving  in  still  other  directions.     Such  cases  are  of 
frequent  occurrence  near  the  commencement  or  during  the  prog- 
ress of  a  great  storm. 

199.  Peculiar  Arrangement  of  Clouds. — Clouds  sometimes  assume 
remarkable  forms,  which  we  can  not  ascribe  to  chance.     Some- 
times cirro-cumulus  clouds  arrange  themselves  in  long  lines, 

stretching  quite  across  the  horizon.  Some- 
times several  such  lines  stretch  across  the 
sky  in  nearly  parallel  directions,  while  oc- 
casionally the  whole  heavens  are  covered 
with  such  bands,  which  seem  to  diverge 
from  one  point  of  the  horizon,  and  con- 
verge to  the  opposite  point.  Such  bands 
generally  point  from  southwest  to  north- 
east, as  shown  in  Fig.  46. 
The  apparent  curvature  of  the  lines  is  the  effect  of  perspective, 
the  bands  being  in  fact  parallel  to  each  other.  The  direction  of 
these  lines  generally  coincides  with  that  of  the  wind,  and  it  has 
been  suspected  that  these  lines  of  cloud  serve  as  conductors  of 
currents  of  electricity,  and  this  may  be  the  agent  which  causes 
the  clouds  to  assume  such  artificial  forms. 

200.  Shadows  of  Clouds. — When  the  atmosphere  is  filled  with  a 


PRECIPITATION  OF  THE  VAPOR  OF  THE  AIR. 


107 


Fig.  4T. 


dense  haze,  the  shadows  of  houses  and  trees  are  often  distinctly 
depicted  upon  the  haze.    So  also  when  the  sky  is  somewhat  hazy, 

the  shadows  of  clouds  can  be 
distinctly  traced  in  the  sky  by 
dark  lines  proceeding  from  the 
sun.  Such  a  haze  most  fre- 
quently prevails  near  the  hori- 
zon, and  hence  these  shadows 
are  most  noticeable  in  that  quar- 
ter of  the  heavens  which  is  be- 
low the  sun.  This  effect  is  of 
common  occurrence  in  summer, 
and  is  known  by  the  name  of  "  the  sun's  drawing  water."  Oc* 
casionally  we  notice  these  shadows  diverging  in  every  direction 
from  the  sun,  not  only  downward,  but  also  laterally  and  even  up- 
ward. These  shadows  are  parallel  bands,  and  the  apparent  di- 
vergence is  the  effect  of  perspective. 

201.  Shadows  after  Sunset. — A  similar  phenomenon  is  fre- 
quently noticed  about  fifteen  minutes  after  sunset,  when  the 
shadows  of  clouds  near  the  horizon  are  projected  upon  the  west- 
ern sky  in  the  form  of  radiant  beams  diverging  from  the  sun. 
These  beams  are  parallel  lines  of  indefinite  length,  but  from  the 
effect  of  perspective  they  seem  to  diverge  from  the  sun,  and  if 
they  could  be  traced  entirely  across  the  sky  they  would,  for  the 
pjg.48.  same  reason,  con- 

verge to  a  point 
directly  opposite 
to  the  sun.  Such 
cases  are  sometimes, 
though  not  very 
frequently  noticed. 
Similar  shadows  are 
sometimes  seen  in 
the  morning  before  sunrise,  and  form  a  conspicuous  feature  of 
the  morning  twilight.  This  effect  is  sometimes  noticed  in  nearly 
every  part  of  the  world.  It  must  have  attracted  the  attention  of 
the  ancient  Greeks,  and  is  thought  to  explain  that  poetic  expres- 
sion, "  the  rosy  fingered  morn." 


108  METEOROLOGY. 


SECTION  V. 

RAIN. 

202.  Origin  of  Rain, — When  a  portion  of  the  vapor  which  ex- 
ists in  the  air  is  condensed,  a  mist  or  cloud  is  formed.    Generally 
this  condensation  proceeds  slowly,  and  the  clouds  which  result 
do  not  furnish  rain.     But  when  this  condensation  takes  place 
with  sufficient  rapidity,  the  small  particles  of  mist  increase  in  di- 
ameter by  the  condensation  of  more  vapor,  and,  forming  drops 
of  considerable  size,  they  descend  to  the  earth  in  a  shower  of 
rain. 

203.  Diameter  and  Velocity  of  Drops  of  Rain.  —  Drops  of  rain 
vary  in  diameter  from  a  quarter  of  an  inch  to  -^th  and  even  ^th 
of  an  inch.     The  velocity  which  they  acquire  in  their  descent  is 
very  small.     A  drop  falling  in  a  vacuum  would  be  continually 
accelerated,  and  at  the  end  of  one  minute  would  have  the  veloci- 
ty of  a  cannon  ball ;  but,  falling  through  the  atmosphere,  the  re- 
sistance increases  with  the  velocity  until  this  resistance  becomes 
equal  to  the  weight  of  the  drop.     When  this  result  takes  place 
there  can  be  no  farther  increase  of  velocity,  and  the  drop  after- 
ward descends  with  a  uniform  motion.     A  drop  of  rain  -^th  of  an 
inch  in  diameter,  by  falling  through  the  atmosphere,  can  not  ac- 
quire a  velocity  exceeding  34  feet  per  second  ;  a  drop  -^Vtli  of  an 
inch  in  diameter  can  only  acquire  a  velocity  of  13  feet  per  second; 
a  drop  T^h  of  an  inch  in  diameter,  a  velocity  of  8  feet  per  second; 
and  a  globule  of  water  10Votn  of  an  inch  in  diameter  can  not  ac- 
quire a  velocity  so  great  as  two  inches  per  second. 

204.  To  Measure  the  Amount  of  Rain. — The  amount  of  rain  which 
falls  from  the  sky  is  measured  by  &pluviameter,  or  rain-gauge.    The 
object  of  the  rain-gauge  is  to  determine  the  average  depth  of  rain 
which  falls  in  a  given  neighborhood.     For  this  purpose  we  catch 
in  a  vessel  the  rain  which  falls  upon  a  limited  space,  as  a  square 
foot,  and  hence  infer  the  amount  which  falls  in  the  neighbor- 
hood.   It  is,  then,  essential  to  the  accuracy  of  our  conclusion  that 
we  catch  all  the  rain  which  falls  within  the  prescribed  limits,  and 
no  more ;  and  also  that  this  amount  be  equal  to  the  average 


PRECIPITATION   OF   THE   VAPOR   OF   THE   AIR. 


109 


depth  which  falls  in  the  vicinity.  To  secure  the 
first  object,  the  edge  of  the  vessel  should  be 
sharp  and  its  sides  upright.  If  the  edge  of  the 
vessel  be  thick,  or  the  sides  be  much  inclined, 
the  rain  which  falls  upon  the  edge,  or  upon  the 
sloping  sides,  will  be  scattered  in  various  direc- 
tions, and  a  part  will  be  wasted.  A  cylinder 
several  inches  deep  is  the  most  convenient  form 
of  gauge.  This  cylinder  may  be  large  or  small. 
They  have  generally  been  made  about  ten 
inches  in  diameter ;  but  a  cylinder  two  inches 
in  diameter,  if  carefully  made,  may  yield  very 
accurate  results.  Fig.  49  shows  the  gauge  em- 
ployed by  the  Smithsonian  Institution.  The 
cylinder  AB  is  two  inches  in  diameter,  and  the 
tube  CD  is  about  half  an  inch  in  diameter. 


205.  Amount  of  Rain  determined.  —  The  amount  of  rain  col- 
lected in  the  gauge  may  be  measured  in  a  tube  properly  gradu- 
ated by  comparing  the  area  of  a  section  of  the  gauge  with  that 
of  the  tube.     Suppose  the  gauge  to  be  a  cylinder  ten  inches  in 
diameter.     Take  a  glass  tube  exactly  one  inch  in  diameter,  and 
graduate  its  side  to  inches  and  tenths,  and  measure  the  rain  in 
this  tube.     One  inch  of  water  in  the  tube  will  correspond  to  one 
hundredth  of  an  inch  in  the  gauge,  and  a  tenth  of  an  inch  in  the 
tube  to  one  thousandth  in  the  gauge.    We  may  thus  easily  meas- 
ure the  depth  of  the  fallen  rain  to  the  accuracy  of  one  thousandth 
of  an  inch.     In  a  similar  manner  we  may  measure  the  depth  of 
the  rain,  whatever  be  the  diameter  or  form  of  the  gauge. 

206.  Proper  Exposure  of  the  Gauge. — In  order  that  the  amount 
of  rain  collected  in  the  gauge  may  be  equal  to  the  average  depth 
which  falls  in  the  vicinity,  a  proper  exposure  is  indispensable, 
and  this  is  sometimes  difficult  to  be  attained.     If  the  gauge  be 
erected  near  a  building  it  is  liable  to  be  affected  by  eddies  or 
currents  of  air,  causing  more  rain  to  fall  on  one  side  of  the  build- 
ing than  on  the  other.     The  most  suitable  place  for  a  rain-gauge 
is  in  an  open  field  remote  from  all  obstructions ;  or,  if  it  must  be 
near  a  building,  a  position  should  be  selected  which  is  least  ex- 
posed to  the  influence  of  eddies. 


110  METEOROLOGY. 

207.  Influence  of  Height  of  the  Gauge. — Two  gauges  placed  near 
each  other,  at  different  elevations,  do  not  generally  collect  the 
same  quantity  of  rain,  the  lower  gauge  usually  showing  the  most 
water.     At  the  Observatory  of  Greenwich,  a  gauge  at  the  sur- 
face of  the  ground  annually  collects  two  thirds  more  rain  than  a 
gauge  elevated  fifty  feet  above  it.     Similar  differences,  but  less  in 
amount,  have  been  observed  at  other  places  in  England,  as  well 
as  in  Paris  and  Philadelphia. 

This  result  has  been  ascribed  to  an  increase  in  the  size  of  the 
drops  as  they  descend  through  a  humid  atmosphere,  the  drops 
being  generally  colder  than  the  surrounding  air.  But  so  rapid 
an  increase  in  the  size  of  a  drop,  amounting  to  two  thirds  in  a 
fall  of  fifty  feet,  is  altogether  incredible.  Moreover,  it  ought 
sometimes  to  happen  that  the  drops  should  diminish  in  size  by 
evaporation  in  traversing  a  warmer  stratum  of  air,  while  observ- 
ation always  indicates  the  greatest  amount  of  rain  in  the  lower 
gauge. 

This  difference  is  probably  caused  by  eddies  formed  in  the  air 
about  the  gauge.  A  portion  of  the  air  which  strikes  against  the 
gauge  glances  up  over  it  and  spreads  out  laterally,  carrying  along 
with  it  the  descending  drops  of  rain,  thus  dispersing  the  drops 
which  would  otherwise  fall  into  the  gauge,  and  diminishing  the 
quantity  of  water  which  it  collects.  These  eddies  are  strongest 
where  the  velocity  of  the  wind  is  the  greatest;  that  is,  they  pro- 
duce the  greatest  effect  at  a  considerable  elevation  above  the 
ground,  where  the  course  of  the  wind  is  unobstructed  by  oppos- 
ing buildings. 

Hence  we  conclude  that  the  best  location  for  a  rain-gauge  is  to 
bury  it  in  the  earth,  with  its  top  rising  but  little  above  the  sur- 
face of  the  ground. 

208.  How  Bain  is  Caused. — Rain  is  but  the  condensed  vapor 
of  the  air,  and  this  condensation  can  only  be  caused  by  cooling 
the  air  below  the  temperature  of  the  dew-point.     This  reduction 
of  temperature  may  be  effected  by  radiation,  or  by  the  contact 
of  warm  air  with  the  cold  surface  of  the  ground,  especially  the 
surface  of  an  elevated  mountain  ;  or  by  the  mingling  of  warm  air 
with  colder  air;  but  these  processes  are  so  gradual,  or  limited  in 
extent,  that  they  probably  never  result  in  any  thing  more  than  a 
fog  or  a  cloud.     In  order  to  produce  an  abundant  rain,  the  air 


PRECIPITATION   OF   THE   VAPOE   OF   THE   AIR.  Ill 

must  be  suddenly  cooled  below  the  dew-point,  and  there  is  no 
mode  in  which  this  can  be  so  readily  accomplished  as  by  forcing 
it  up  to  an  elevation  of  one  or  two  miles  above  the  earth's  sur- 
face. The  temperature  of  the  air  sinks  about  thirty -five  degrees 
in  two  miles  of  elevation;  and  if  air  from  the  earth's  surface  should 
be  forced  up  to  this  height,  a  large  portion  of  the  vapor  which  is 
carried  up  with  the  air  must  be  condensed.  Such  an  effect  may 
be  produced  by  an  interposed  mountain.  Generally,  however,  it 
is  the  result  of  unusual  heat  or  unusual  moisture  in  the  lower 
strata  of  the  atmosphere. 

&  209.  Button's  Theory  of  Rain.— In  1784,  Dr.  Hutton,  of  Edin- 
burg,  proposed  a  theory  of  rain,  which  has  acquired  great  celebri- 
ty. This  theory  is  founded  upon  the  following  principle:  When 
two  masses  of  air  of  different  temperatures,  and  both  saturated 
with  vapor,  are  mingled  together,  the  temperature  of  the  mixture 
is  too  low  to  contain  all  the  vapor  of  the  combined  masses.  This 
excess  of  moisture  must  therefore  be  discharged  in  the  form  of 
rain. 

Suppose,  for  example,  there  are  two  masses  of  air  having  the 
temperatures  of  60°  and  80°,  and  that  each  is  saturated  with 
moisture.  The  elastic  force  of  vapor  at  these  temperatures  is 
0.518  and  1.023 ;  the  mean  of  the  two  being  0.770  inch.  Sup- 
pose the  mixture  to  have  a  temperature  of  70°,  at  which  tempera- 
ture the  elastic  force  of  vapor  is  0.733  inch.  The  difference  is 
0.037  inch  of  mercury,  or  0.503  inch  of  water,  and  this,  it  is  claim- 
ed, is  the  amount  of  water  that  should  be  precipitated  the  moment 
these  two  masses  of  air  are  perfectty  mingled.  A  similar  result 
should  take  place  if  the  two  masses  of  air  contain  considerable 
moisture,  but  without  being  saturated. 

It  is  objected  to  this  theory  that  it  is  impossible  to  mingle  to- 
gether two  large  masses  of  air  of  different  temperatures,  except 
very  slowly,  and  hence  the  resulting  precipitation  can  not  be  con- 
siderable. Moreover,  the  latent  heat  evolved  in  the  condensation 
of  the  vapor  would  raise  the  temperature  of  the  mixture,  so  that 
a  less  quantity  of  water  than  that  above  supposed  would  be  pre- 
cipitated. Such  a  mixture  might,  therefore,  give  rise  to  a  cloud, 
but  never  to  a  copious  shower. 

210.  Distribution  of  Rain  over  the  Earth's  Surface. — The  fall  of 


PRECIPITATION   OF  THE   VAPOR  OF  THE   AIR.  US 

rain  is  very  unequally  distributed  over  the  earth's  surface,  vary- 
ing from  zero  to  a  depth  of  fifty  feet  in  a  year.  The  amount  of 
rain  is  affected  by  the  latitude  of  the  place ;  by  its  elevation 
above  the  sea ;  by  the  proximity  and  course  of  chains  of  mount- 
ains; the  proximity  and  configuration  of  the  coast,  as  well  as 
by  the  direction  of  the  prevalent  winds.  The  preceding  chart 
shows  the  average  annual  distribution  of  rain  over  the  earth's 
surface;  the  lightest  shade  indicating  an  annual  rain-fall  of  less 
than  10  inches ;  the  second  shade  indicating  a  rain-fall  from  10  to 
25  inches ;  the  third  shade  from  25  to  50  inches ;  and  the  deepest 
shade  indicating  a  rain-fall  exceeding  50  inches  annually. 

211.  Equatorial  Rain-belt. — Since  the  average  amount  of  vapor 
present  in  the  air  near  the  equator  is  much  greater  than  it  is  in 
high  latitudes,  we  might  expect  to  find  a  greater  average  rain-fall 
in  the  former  region  than  in  the  latter.  Such  is  generally  found 
to  be  the  case.  Near  the  Atlantic  coast  of  the  American  conti- 
nent we  find  a  rain-fall  of  more  than  50  inches,  extending  (with  a 
few  interruptions)  from  latitude  35°  North  to  latitude  33°  South. 
The  area  of  50  inches'  rain-fall  includes  all  the  southern  part  of 
ihe  United  States  as  far  westward  as  Texas;  it  includes  Central 
America,  the  West  India  Islands,  and  the  principal  part  of  South 
America  east  of  the  Andes ;  it  stretches  entirely  across  the  conti- 
nent of  Africa  in  a  belt  whose  average  breadth  is  about  1000 
miles ;  it  includes  all  the  islands  of  the  East  Indian  Archipelago, 
with  portions  of  the  southern  coast  of  Asia  and  the  northern 
coast  of  Australia.  So  far,  then,  as  concerns  the  continents  and 
islands,  we  find  a  continuous  belt  of  rain  exceeding  50  inches  an- 
nually, surrounding  the  globe  in  the  neighborhood  of  the  equa- 
tor, and  having  an  average  breadth  considerably  exceeding  1000 
miles. 

This  belt  of  50  inches'  rain -fall  includes  extensive  regions 
where  the  rain-fall  amounts  to  75  inches  and  upwards.  A  rain- 
fall of  more  than  75  inches  is  found  on  most  of  the  West  India 
Islands ;  also  along  a  considerable  part  of  the  Atlantic  coast  of 
Central  America  and  South  America;  along  the  eastern  slope  of 
the  Andes  as  far  south  as  latitude  20° ;  also  on  the  Atlantic  coast 
of  Africa  near  the  equator;  on  the  mountains  of  Abyssinia; 
throughout  most  of  the  islands  of  the  East  Indian  Archipelago; 
and  portions  of  the  southern  coast  of  Asia 

H 


114  METEOROLOGY. 

In  the  higher  latitudes  there  are  districts  of  comparatively  lim- 
ited extent  where  the  annual  rain-fall  exceeds  50  inches;  but  these 
districts  are  in  the  neighborhood  of  mountains,  and  they  do  not 
form  a  continuous  belt  surrounding  the  globe,  Kke  the  belt  above 
described  in  the  torrid  zone. 

The  amount  of  rain-fall  over  the  ocean  is  very  imperfectly 
known,  since  it  is  difficult  to  measure  the  rain-fall  on  board  of  a 
ship  at  sea. 

212.  Number  of  Rainy  Days. — The  number  of  rainy  days  in 
the  year  generally  increases  from  the  equator  to  the  polar  regions, 
and  diminishes  from  the  coasts  to  the  interior  of  the  continents; 
but  it  is  also  affected  by  other  circumstances  which  influence  the 
total  amount  of  rain-fall.     Over  the  Atlantic  Ocean,  in  latitude 
50°,  the  number  of  rainy  days  is  about  the  same  as  at  the  equator, 
and  is  double  of  what  it  is  from  latitude  10°  to  30°.     In  New 
England  the  average  number  of  rainy  days  in  a  year  is  90 ; 
throughout  most  of  the  western  states  it  is  somewhat  greater;  in 
California  it  is  only  50;  in  Oregon  it  is  130;  and  in  Alaska  it  is 
235.     In  Great  Britain  the  average  number  of  rainy  days  in  a 
year  is  156;   throughout  most  of  Europe  it  is  somewhat  less; 
while  in  Siberia  it  is  only  60. 

213.  Influence  of  Elevation  above  the  Sea. — The  annual  fall  of 
rain  is  uniformly  greater  on  mountains  of  moderate   elevation 
than  it  is  at  the  level  of  the  sea;  and  at  a  certain  height  the  fall 
is  from  two  to  three  times  as  great  as  it  is  near  the  base  of  the 
mountain.     On  the  island  of  Guadeloupe,  in  latitude  16°,  near 
the  summit  of  a  mountain  of  5000  feet  elevation,  the  fall  of  rain 
in  1828  was  292  inches,  while  near  the  base  of  the  mountain  the 
fall  was  only  127  inches.     This  difference  is  not  due  to  the  cold- 
ness of  the  mountain.     An  equal  and  probably  a  greater  effect 
would  be  produced  by  a  volcanic  mountain  whose  surface  was 
covered  with  melted  lava.     When  a  current  of  air  meets  an  in- 
terposed mountain,  it  is  forced  up  the  side  of  the  mountain  ;  that 
is,  it  is  elevated  above  the  earth's  surface  into  a  colder  region,  and 
its  vapor  is  precipitated  by  the  cold  of  elevation. 

We  find  the  same  principle  exemplified  wherever  there  are 
high  mountains.  Along  the  western  coast  of  Hindostan  runs  a 
range  of  mountains  whose  summits  are  deluged  with  rain,  while 


PRECIPITATION   OF  THE   VAPOR  OF   THE   AIR.  115 

near  their  western  base  the  amount  of  rain  is  by  no  means  ex- 
traordinary, and  on  their  eastern  side  the  fall  is  less  than  one  third 
of  the  average  for  the  same  latitude.    At  Bombay,  on  the  western 
Flg  51  side   of  the   mountain, 

the  average  annual  fall 
of  rain  is  78  inches;  at 
the  elevation  of  4500 
feet  the  average  fall  is 
254  inches,  and  in  1842 

the  fall  amounted  to  305  inches ;  while  at  Poonah,  on  the  eastern 
side  of  the  mountain,  the  average  fall  is  only  23  inches.  This 
rain  falls  almost  wholly  from  June  to  October,  during  the  preva- 
lence of  the  southwest  monsoon.  The  warm  and  moist  air  from 
the  ocean,  encountering  this  range  of  mountains,  is  elevated  high 
above  the  surface  of  the  sea,  by  which  means  it  is  cooled,  and  its 
vapor  is  condensed  over  the  summit  of  the  mountain.  When 
this  air  descends  on  the  eastern  side  of  the  mountain  it  is  a  dry 
air,  and  has  but  little  vapor  remaining  to  be  precipitated. 

A  similar  effect  takes  place  on  the  southern  slope  of  the  Him- 
alaya Mountains,  about  300  miles  north  of  Calcutta,  where,  at  an 
elevation  of  4500  feet,  the  fall  of  rain  in  1851  was  610  inches,  all 
of  which  fell  from  April  to  October,  during  the  prevalence  of  the 
southwest  monsoon. 

On  the  summit  of  Mt.  Washington  the  average  rain-fall,  as  de- 
duced from  the  observations  of  eight  years,  is  78  inches;  which 
is  about  double  the  average  rain-fall  for  New  England. 

214.  Maximum  Fall  of  Rain. — The  increased  fall  of  rain  upon 
mountains  attains  its  maximum  at  a  certain  elevation,  and  above 
that  point  the  fall  of  rain  decreases  as  we  ascend.     The  elevation 
at  which  the  fall  of  rain  is  greatest  is  not  every  where  the  same. 
In  India  it  is  about  4500  feet,  while  in  Great  Britain  it  is  about 
1900  feet. 

215.  Influence  of  Proximity  to  a  Mountain. — Sometimes  the  mere 
proximity  to  a  mountain  causes  more  rain  to  fall  at  the  level  of 
the  sea  than  is  usually  found  in  the  same  latitude.    Thus,  at  Vera 
Cruz,  278  inches  of  rain  have  been  known  to  fall  in  a  single  year; 
and  the  mean  annual  fall  is  185  inches,  which  is  fully  double  the 
average  amount  for  the  Gulf  of  Mexico.     This  result  is  to  be  as- 


116  METEOROLOGY. 

cribed  to  the  high  mountains  on  the  west  side  of  Vera  Cruz,  by 
which  the  warm  and  moist  air  from  the  Gulf  is  forced  up  to  a 
great  height,  and  its  vapor  is  condensed  by  the  cold  of  elevation, 
and  this  influence  is  not  confined  to  the  immediate  vicinity  of  the 
mountain,  but  extends  to  some  distance  beyond  its  base. 

So,  also,  on  the  Northwest  Coast  of  America,  near  latitude  60", 
for  a  similar  reason,  the  annual  fall  of  rain  is  90  inches,  which  is 
at  least  four  times  the  average  for  other  parts  of  the  globe  in  the 
same  latitude. 

For  a  like  reason,  on  the  coast  of  Norway,  in  latitude  60°,  the 
annual  fall  of  rain  is  more  than  80  inches. 

216.  Influence  of  Proximity  to  the  Ocean. — An  increase  of  rain 
usually  results  from  mere  proximity  to  the  ocean,  even  where 
there  are  no  mountains,  especially  if  the  prevalent  winds  come 
from  the  sea.     This  effect  is  most  noticeable  near  the  coast,  and 
goes  on  diminishing  as  we  proceed  toward  the  interior  of  a  conti- 
nent.    Thus,  in  Europe,  near  the  Atlantic  coast,  the  fall  of  rain 
varies  from  30  to  40  inches;  in  Central  Europe  it  seldom  exceeds 
20  inches ;  while  throughout  a  large  part  of  Russia  it  is  only  15 
inches,  and  in  Northern  Asia  it  is  still  less. 

Similar  results,  but  somewhat  more  complicated,  are  found  in 
the  United  States.  On  the  Atlantic  coast,  near  the  parallel  of 
45°,  the  annual  fall  of  rain  is  40  inches;  in  Michigan  it  is  about 
30  inches;  in  Minnesota  25  inches;  and  near  the  Missouri  River, 
on  the  same  parallel,  it  is  only  15  inches. 

217.  Influence  of  Winds.  —  Along  the   Atlantic   coast  of  the 
United  States,  rain  occurs  most  frequently  with  the  wind  from 
the  northeast.     Out  of  one  hundred  cases  of  rain  or  snow  re- 
corded at  New  Haven,  the  average  number  occurring  with  the 
different  winds  is  as  follows : 

N.        N.E.        E.        S.E.        S.        S.W.       W.        N.W. 

8          37          6          19         7          15  1  7 

Storms  at  New  Haven  generally  begin  with  an  easterly  wind  and 
end  with  a  westerly  wind,  so  that  the  same  storm  is  attended  by 
both  winds ;  but  as  the  rain  or  snow  with  the  first  wind  general- 
ly continues  longest,  the  easterly  wind  is  recorded  as  accompany- 
ing rain  at  a  greater  number  of  the  regular  hours  of  observation. 

Throughout  most  of  the  interior  of  the  United  States,  the  prin- 


PRECIPITATION   OF  THE   VAPOR   OF   THE   AIR. 


117 


cipal  part  of  the  rain  comes  with  a  westerly  wind.     At  Cincin- 
nati, out  of  one  hundred  cases  of  rain  or  snow,  the  average  num- 
ber occurring  with  the  different  winds  is  as  follows : 
N.        N.E.        E.        S.E.        S.        S.W.        W.        N.W. 
2  10          1  9         10          25  18          25 

In  Central  Europe  about  three  fourths  of  all  the  rain  occurs 
with  a  westerly  wind. 

218.  Annual  Fall  of  Rain  at  different  Places.  —  To  obtain  the 
mean  fall  of  rain  at  any  place  requires  observations  continued 
for  a  considerable  number  of  years,  for  it  not  unfrequently  hap- 
pens that  the  rain  of  one  year  is  double  that  of  some  other 
year  at  the  same  place.  The  following  table  shows  approxi- 
mately the  average  annual  fall  of  rain  for  different  parts  of  the 
United  States: 


Inches. 

Alabama  and  Louisiana  .     52 

Oregon 50 

Florida 50 

Virginia  and  the  Carolinas  45 
Tennessee  and  Kentucky     45 

Georgia 45 

Arkansas  and  Missouri    .     44 
Maryland  and  Pennsylvania  40 


Ohio 

New  England     .     .     .     . 

New  York 

Michigan  and  Wisconsin 
Iowa  and  Kansas    . 


Inches. 

40 
40 
36 
31 
31 


Texas 31 

California 20 

New  Mexico .  14 


219.  Distribution  of  Rain  throughout  the  Year.  —  Throughout 
most  of  the  United  States  east  of  the  Eocky  Mountains,  the  rain 
is  pretty  equally  distributed  through  the  different  months  of  the 
year,  but  the  rain  of  summer  is  every  where  somewhat  greater 
than  the  rain  of  winter,  including  the  melted  snow.  In  New  En- 
gland the  difference  between  the  rain  for  these  two  seasons  is  less 
than  10  per  cent. ;  in  the  State  of  New  York  it  is  nearly  50  per 
cent. ;  in  Virginia  and  the  Carolinas  it  is  100  per  cent. ;  in  Flori- 
da it  is  200  per  cent. ;  in  Texas  it  is  75  per  cent. ;  in  Ohio  it  is  25 
per  cent. ;  in  Michigan  and  Wisconsin  it  is  140  per  cent. ;  while  in 
Iowa  and  Kansas  it  is  300  per  cent ;  that  is,  the  fall  of  rain  in 
summer  is  four  times  as  great  as  it  is  in  winter.  On  the  Pacific 
coast  this  law  is  reversed.  In  California  the  rain  of  winter  is 
more  than  twenty  times  as  great  as  that  of  summer,  and  in  Ore- 
gon it  is  seven  times  as  great.  See  Table  XXIX. 


118  METEOROLOGY. 

220.  Rainy  Season  and  Dry  Season.  —  When  the  rain  is  very 
unequally  distributed  through  the  different  months,  the  year  i3 
naturally   divided  into   the   rainy   season   and  the   dry  season. 
Throughout  most  of  California  but  little  rain  falls  except  during 
the  six  colder  months,  and  during  the  four  months  from  June  to 
September  rain  is  almost  unknown.     No  rain  falls  during  the 
summer  months,  when  the  wind  blows  almost  uninterruptedly 
from  the  southwest,  because  this  air  comes  from  a  colder  ocean, 
and,  passing  over  the  heated  land,  its  vapor  is  not  condensed  until 
it  meets  the  Nevada  Mountains,  on  the  eastern  margin  of  Call' 
fornia. 

Wherever  the  direction  of  the  prevalent  wind  changes  greatly 
with  the  season  of  the  year,  we  generally  find  the  rain  unequally 
distributed  through  the  different  months.  On  the  ivest  coast  of 
Hindostan,  nearly  all  the  rain  falls  from  April  to  September,  dur 
ing  the  prevalence  of  the  southwest  monsoon ;  but  during  the 
other  half  of  the  year,  the  winds  coming  from  the  northeast  have 
already  passed  over  high  mountains,  where  they  have  lost  their 
moisture,  and  descend  to  the  earth  as  dry  winds,  which  often  fur- 
nish no  rain  for  months  in  succession. 

On  the  east  coast  of  Hindostan,  almost  no  rain  faUs  during  the 
prevalence  of  the  southwest  monsoon,  but  abundant  rains  occur 
during  the  prevalence  of  the  northeast  monsoon,  when  the  warm 
air  from  the  Bay  of  Bengal  has  a  higher  temperature  than  the 
land. 

A  similar  inequality  occurs  at  many  places  in  tropical  America. 
At  Vera  Cruz  almost  the  entire  fall  of  rain  occurs  from  May  to 
October,  when  the  winds  are  easterly  ;  but  during  the  rest  of  the 
year  the  winds  are  northwesterly,  and  several  months  will  some- 
times pass  without  a  drop  of  rain. 

At  some  places  near  the  equator  there  are  two  rainy  periods  of 
the  year,  the  maxima  occurring  in  June  and  December. 

221.  Greatest  Fall  of  Rain. — There  are  certain  portions  of  the 
globe  which  are  habitually,  and  others  occasionally  deluged  with 
rain.     On  the  southern  slope  of  the  Himalaya  Mountains,  at  the 
height  of  4500  feet,  in  latitude  25°,  there  have  been  registered  in 
a  single  year  610  inches  of  rain ;  and  of  this,  147  inches  fell  in 
the  month  of  June.     At  a  station  in  latitude  18°,  near  the  west- 
ern coast  of  Hindostan,  the  average  fall  for  fifteen  years  has  been 


PRECIPITATION   OF  THE   VAPOR  OF  THE   AIR.  119 

254  inches.  In  the  northwestern  part  of  England,  at  the  height 
of  1300  feet,  the  average  annual  fall  of  rain  is  146  inches,  while 
at  London  the  annual  fall  is  only  20  inches.  At  Vera  Cruz  the 
annual  fall  is  183  inches,  and  60  inches  have  been  recorded  in  a 
single  month.  See  Table  XXXI. 

222.  Remarkable  Showers. — Throughout  most  of  the  United 
States  the  rain  which  falls  in  one  day  rarely  amounts  to  one  inch, 
but  occasionally  the  fall  is  much  more  remarkable.    Thus,  at  Flat- 
bush,  Long  Island,  on  the  22d  of  August,  1843,  nine  inches  of  rain 
fell  in  eight  hours;  at  Catskill,  New  York,  on  the  26th  of  July, 
1819,  fifteen  inches  of  rain  fell  in  six  hours;  at  Wilmington,  Del- 
aware, on  the  29th  of  July,  1834,  five  inches  of  rain  fell  in  two 
and  a  half  hours;  and  at  Fairfield,  Ohio,  on  the  12th  of  August, 
1861,  eight  inches  of  rain  fell  in  eleven  hours. 

In  India  fifteen  inches  of  rain  have  fallen  in  a  single  day,  while 
at  several  places  in  the  vicinity  of  Switzerland  thirty  inches  of 
rain  have  been  reported  to  fall  in  a  single  day. 

It  is  not  supposed  that  in  any  of  these  cases  the  amount  of  rain 
was  measured  with  absolute  precision ;  but  that  the  fall  was  very 
unusual  was  evident  from  the  aspect  of  the  country  after  the 
storm. 

Bains  so  remarkable  are  necessarily  quite  limited  in  extent,  for, 
if  every  particle  of  moisture  in  the  atmosphere  were  precipitated, 
it  would  cover  the  entire  globe  to  a  depth  of  less  than  four  inches. 
This  result  is  obtained  as  follows :  The  average  temperature  of 
the  entire  surface  of  the  globe  is  estimated  at  58°,  and  the  aver- 
age dew-point  at  51°.  At  this  temperature  vapor  will  sustain  a 
column  of  mercury  0.374  inch  in  height.  The  weights  of  equal 
volumes  of  aqueous  vapor  and  air  at  the  same  temperature  and 
pressure  are  as  5  to  8  nearly,  and  the  specific  gravity  of  mercury 
is  13.6.  Hence  vapor  at  51°,  reduced  to  water,  becomes  0.374  x 
13.6  x  0.624,  which  equals  3.17  inches. 

At  the  close  of  a  long  rain-storm  it  is  not  uncommon  for  the 
air  to  contain  more  moisture  than  it  did  at  its  commencement. 
Hence  we  must  conclude  that  the  rain  which  falls  in  these  re- 
markable showers  is  derived  from  moist  air  drawn  from  remote 
places. 

223.  Deserts. — There  are  large  portions  of  the  earth's  surface 


120  JVTJETEOROLOGY. 

where  rain  is  almost  entirely  unknown,  viz.,  the  interior  of  Africa 
between  the  parallels  of  20°  and  30°,  including  most  of  Egypt-, 
also  a  considerable  portion  of  Arabia  and  Persia ;  the  great  des- 
ert of  Gobi,  on  the  northeast  side  of  the  Himalaya  Mountains, 
with  portions  of  Peru,  and  California. 

There  are  also  other  districts  where  the  amount  of  rain  does 
not  exceed  one  tenth  of  that  which  is  found  elsewhere  in  the  same 
latitude,  such  as  Lower  California,  where  the  annual  fall  of  rain  is 
only  three  inches ;  also  the  northern  coast  of  Africa,  Lower  Egypt, 
and  Persia.  See  Table  XXX. 

224.  Cause  of  the  African  Desert. — The  Great  Desert  of  Africa 
lies  near  the  northern  limit  of  the  trade  winds,  where,  as  we  have 
already  seen,  the  causes  which  produce  rain  act  with  the  least  en- 
ergy.    This  desert  is  an  immense  sandy  plain,  with  a  range  of 
mountains  near  its  northern,  as  well   as  its  southern  border. 
When  the  N.E.  trade  wind  first  strikes  the  continent  of  Africa,  a 
portion  of  the  vapor  is  condensed  on  the  northern  mountains.    As 
the  wind  proceeds  southward,  it  advances  toward  a  warmer  lati- 
tude, which  has  a  greater  capacity  for  moisture;  and  there  are  no 
mountains  or  opposing  winds  to  force  the  air  up  above  the  earth's 
surface  until  we  approach  the  parallel  of  10°,  where  we  find  a  long 
chain  of  mountains,  over  which  the  vapor  is  condensed  in  copious 
rains.      The  heat  which  is  liberated  in  the  condensation  of  this 
vapor  is  one  cause  of  the  steady  trade  winds,  and  the  absence  of 
rain  over  the  Desert.     Here  and  there  in  the  midst  of  the  Desert 
is  found  a  high  peak,  or  small  mountain,  and  here  rain  is  occasion- 
ally seen. 

Similar  considerations  explain  the  small  amount  of  rain  in 
Egypt  and  Arabia. 

225.  Great  Desert  of  Gobi,  etc.  —  The  great  desert  of  Gobi  is 
caused  by  the  Himalaya  Mountains.     Here  the  prevalent  winds 
are  from  the  S.W.,  and,  having  just  passed  over  the  mountains, 
they  have  lost  nearly  all  their  vapor,  that  is,  they  are  extremely 
dry  winds,  having  little  moisture  to  be  precipitated. 

Peru  is  situated  within  the  region  of  the  S.E.  trade  winds,  which, 
on  meeting  the  Andes,  are  forced  up  to  such  an  elevation  that 
their  moisture  is  nearly  all  condensed,  and  they  descend  on  the 
Pacific  side  as  dry  winds,  and  have  no  moisture  which  can  be  cou- 


PRECIPITATION   OF   THE   VAPOR  OF  THE   AIR.  121 

densed  at  the  temperature  which  prevails  in  Peru.  The  princi- 
pal tributaries  of  the  Amazon  are  fed  by  the  rains  which  fall  on 
the  windward  side  of  the  Andes. 

Between  the  two  great  mountain  ranges,  the  Sierra  Nevadas 
and  the  Rocky  Mountains,  comprehending  portions  of  Utah,  New 
Mexico,  and  California,  is  a  region  which  is  almost  entirely  desti- 
tute of  rain.  Throughout  this  region,  whether  the  wind  blows 
from  the  east  or  the  west,  it  has  lost  most  of  its  vapor  by  passing 
over  the  mountains.  It  is  therefore  a  dry  air,  and  has  but  little 
vapor  to  be  precipitated. 

So,  also,  on  the  east  side  of  the  Kooky  Mountains,  the  prevalent 
winds,  being  westerly,  have  lost  their  vapor  by  passing  over  the 
mountains,  and  the  country  is  a  barren  desert,  almost  without  rain. 

226.  Rain  without  Clouds. — Ordinarily  clouds  seem  to  be  the 
reservoirs  from  which  the  rain  descends,  but  rain  has  been  known 
to  fall  when  no  cloud  could  be  seen  near  the  zenith,  and  even  at 
times  when  no  cloud  appeared  above  the  horizon.     Thus,  on  the 
23d  of  April/1800,  at  9  P.M.,  rain  fell  for  twenty  minutes  at  Phil- 
adelphia, although  the  heavens  immediately  overhead  appeared 
perfectly  clear,  and  the  stars  shone  with  undiminished  lustre.  Not 
a  cloud  could  be  seen  within  15°  of  the  zenith.     Also  on  the  9th 
of  August,  1837,  a  shower  fell  at  Geneva,  Switzerland-,  which  last- 
ed two  or  threp  minutes,  although  the  sky  was  cloudless.     Many 
similar  cases  have  been  observed  in  other  parts  of  the  world. 

227.  Rain  from  Clouds  not  in  the  Zenith. — It  is  probable  that  in 
some  cases  rain  reaches  the  earth's  surface  from  clouds  removed 
several  degrees  from  the  zenith.     The  path  of  a  rain-drop  often 
makes  an  angle  with  the  vertical  greater  even  than  45°,  and  rain 
might  therefore  reach  the  earth  from  a  cloud  removed  20°  or  30°, 
and  perhaps  even  farther  from  the  zenith,  especially  if  there  pre- 
vailed near  the  earth's  surface  a  fresh  breeze,  blowing  in  a  direc- 
tion different  from  that  of  the  current  which  conveys  the  cloud. 
This  principle  will  probably  explain   some   of  the  cases  which 
have  been  reported;  but  there  are  other  cases  in  which  it  is  said 
that  rain  has  fallen,  although  no  cloud  was  visible  above  the  hor- 
izon. 

228.  Rain  from  Translucent    Clouds.— -It  is  probable   that,  in 


122  METEOROLOGY. 

these  cases  of  remarkable  rain-falls,  although  the  sky  was  free 
from  dense  clouds,  such  as  entirely  conceal  the  stars,  it  was  not 
entirely  free  from  a  haziness,  which  is,  indeed,  nothing  else  than 
a  cloud  so  thin  as  to  allow  the  brighter  stars  to  shine  through  it. 
The  partial  transparency  of  such  a  cloud  may  be  due  to  the  small 
number  and  large  size  of  the  rain-drops. 

Pure  water  is  nearly  transparent,  and  a  fog  is  opaque,  simply 
on  account  of  the  minuteness  and  consequent  multitude  of  the 
condensed  particles.  A  certain  amount  of  light  is  reflected  from 
the  surface  of  each  particle,  and  in  a  fog  the  number  of  reflecting 
surfaces  is  so  great  that  a  beam  of  light  is  wholly  reflected  before 
it  can  penetrate  through  the  mass.  But  if  the  amount  of  water 
which  composes  a  fog  were  all  collected  in  a  few  large  drops,  the 
number  of  reflecting  surfaces  would  be  comparatively  small ;  that 
is,  they  would  but  slightly  affect  the  transparency  of  the  air.  It 
is  probable,  therefore,  that  when  rain  falls  from  a  cloudless  sky, 
the  vapor  is  condensed  in  a  few  large  drops,  instead  of  a  multi- 
tude of  minute  ones.  This  condensation  probably  takes  place 
with  great  suddenness  in  the  lower  stratum  of  the  air,  which  was 
previously  saturated  with  moistmv. 

229.  Snow  from  a  Cloudless  Sky. — In  the  polar  regions  a  fine 
snow  sometimes  falls  from  a  cloudless  sky.     So,  also,  in  New  En- 
gland, during  a  period  of  intense  cold,  we  sometimes  see  flakes  of 
snow  descending  from  the  sky,  although  there  is  no  cloud  suffi- 
cient to  obscure  the  sun  or  moon,  or  even  the  light  of  the  bright- 
er stars.     In  such  cases,  the  vapor  rising  from  the  earth  is  prob- 
ably condensed  before  it  attains  a  great  elevation,  and  both  the 
thickness  and  density  of  the  cloud  are  quite  small. 

SECTION  VI. 

SNOW. 

230.  How  Flakes  of.  Snow  are  formed. — When  the  vapor  of  the 
air  is  precipitated  at  a  very  low  temperature,  the  vapor  is  con- 
densed in  the  solid  state,  without  passing  through  the  condition 
of  a  liquid,  and  generally  assumes  the  crystalline  form.     These 
minute  crystals  of  ice  attach  themselves  to  each  other  and  form 
flakes  of  snow,  which  descend  very  slowly  to  the  earth.     When 
the  lower  stratum  of  the  air  is  much  above  32°,  the  flakes  of  snow 


PRECIPITATION  OF  THE  VAPOR  OF  THE  AIR.  123 

melt  before  they  reach  the  ground,  so  that  rain  may  frequently  be 
seen  falling  on  an  open  plain,  while  from  the  same  cloud  snow  is 
falling  upon  a  neighboring  mountain. 

During  the  severe  cold  of  winter  we  may  frequently  witness 
snow  produced  artificially.  When  a  large  number  of  people  are 
assembled  in  the  same  hall,  and  the  room  being  uncomfortably 
v,Tarm,  a  window  is  opened,  the  warm  air  of  the  room  flows  rapid- 
ly outward,  and  its  vapor  is  condensed,  and  it  sometimes  falls  to 
the  ground  in  the  form  of  flakes  of  snow  of  extreme  delicacy. 

231.  Where  Snow  Falls. — Within  the  torrid  zone  snow  is  al- 
most never  seen,  except  on  elevated  mountains,  because  near  the 
level  of  the  sea  the  temperature  is  above  the  freezing  point.    For 
a  similar  reason,  in  the  middle  latitudes,  the  fall  of  snow  occurs 
only  in  winter,  while  in  the  polar  regions  nearly  all  the  moisture 
which  is  precipitated  descends  to  the  earth  in  the  form  of  snow. 

The  zone  within  which  snow  never  falls  is  determined  not  so 
much  by  the  mean  temperature  of  the  year,  or  the  mean  temper- 
ature of  the  coldest  month,  as  by  the  temperature  of  the  coldest 
day  of  winter.  At  all  places  where  the  thermometer  in  winter 
sinks  much  below  32°,  snow  may  occasionally  fall.  The  bounda- 
ry of  the  zone  within  which  snow  does  not  fall,  except  in  a  few 
very  rare  cases,  is  an  undulating  line  crossing  the  Pacific  coast 
of  America  near  lat.  39°,  and  the  Atlantic  coast  near  lat.  35° ;  it 
passes  near  Gibraltar  in  lat.  36°,  and  on  the  coast  of  China  de- 
scends to  lat.  24°,  which  is  but  a  little  north  of  Canton. 

A  slight  fall  of  snow  occasionally  occurs  at  San  Francisco,  Cali- 
fornia ;  it  occasionally  falls  at  New  Orleans,  and  also  at  Galves- 
ton,  lat.  29°;  and  snow  sufficient  for  sleighing  has  been  known  at 
Charleston,  S.  C.  Snow  has  also  been  known  to  fall  at  Canton, 
within  the  torrid  zone,  to  the  depth  of  four  inches. 

232.  Annual  Amount  of  Snow. — The  amount  of  snow  which 
falls  in  a  year  varies  in  different  localities  from  zero  to  twelve  feet. 
In  Spitzbergen  the  annual  fall  of  snow  is  from  three  to  five  feet. 
In  the  northeastern  part  of  Nova  Scotia  the  annual  fall  of  snov; 
is  twelve  feet.     In  the  State  of  Maine  it  is  seven  and  a  half  feet, 
and  some  years  it  exceeds  twelve  feet.     In  Yermont  and  New 
Hampshire  the  annual  fall  is  six  feet,  and  in  some  places  ten.feet. 
In  Central  Massachusetts  the  annual  fall  is  four  and  a  half  feet,  and 


124  METEOROLOGY. 

the  snow  bas  been  known  to  lie  five  feet  on  a  level.  In  Connecti- 
cut the  average  fall  is  three  and  a  half  feet;  in  New  Jersey,  two 
and  a  half  feet;  in  Southern  Ohio,  one  foot  and  a  half;  and  in 
Iowa,  one  foot. 

Snow  recently  fallen  has  a  very  small  specific  gravity,  for  a 
foot  of  snow,  when  melted,  furnishes  only  one  inch  of  water. 

233.  Form  of  Snow-flakes. — Crystals  of  ice  generally  exhibit 
the  form  of  long  needles  or  spiculre,  each  being  a  slender  prism 
with  angles  of  120°.  These  crystals  are  often  seen  in  great  per- 
fection in  hoar-frost.  Flakes  of  snow  generally  consist  of  combi- 
nations of  spicula3  and  of  thin  plates  or  laminae  of  ice,  which  usu- 
ally present  angles  of  60°  or  120°.  Sometimes  we  find  simply  six 
spicula3  combined  in  angles  of  60°,  forming  a  star  with  six  rays. 
Sometimes  to  each  of  these  spiculae  are  attached  shorter  spiculse, 
also  inclined  at  angles  of  60°,  in  number  amounting  to  2,  4,  6,  etc., 
up  to  a  dozen  or  more,  forming  a  perfectly  sj'mmetrical  figure 
bearing  some  resemblance  to  a  flower  of  great  complexity.  See 
the  first  six  forms  in  Fig.  52. 

Fig.  52. 


PRECIPITATION  OF  THE  VAPOR  OF  THE  AIR.  125 

Sometimes  we  find  a  simple  lamina  of  ice,  in  which  case  the 
form  is  usually  that  of  a  regular  hexagon,  which  sometimes  has 
the  appearance  of  being  composed  of  equilateral  triangles.  Some- 
times ice  spiculae  are  attached  to  the  angles  of  the  hexagon  ;  some- 
times attached  to  the  angles  of  a  central  hexagon  we  find  six 
smaller  hexagons,  or  perhaps  rhomboids  composed  of  two  equi- 
lateral triangles.  Sometimes  the  central  figure  consists  of  six 
such  rhomboids,  with  ice  spiculae  or  other  rhomboids  attached  to 
the  angles. 

Sometimes  the  flakes  present  forms  which  can  not  apparently 
be  resolved  into  any  of  the  preceding  elements. 

Several  hundred  different  forms  of  snow  crystals  have  been  ob- 
served and  figured.  Fig.  52  presents  a  specimen  of  the  simplest 
forms,  and  also  of  the  most  complicated.  These  crystals  are  seen 
in  their  greatest  perfection  when  the  air  is  tranquil,  cold,  and  dry. 

234.  Size  of  Snow-flakes. — Snow-flakes  vary  in  size,  according 
to  the  temperature  at  which  they  are  formed.     If  formed  at  a  very 
low  temperature,  their  diameter  is  often  less  than  one  tenth  of  an 
inch ;  when  formed  near  the  temperature  of  32°,  they  are  some- 
times found  one  inch  in  diameter. 

235.  Natural  Snoiu-balls. — Sometimes  a  vast  number  of  snow- 
flakes  attach  themselves  together,  and  descend  to  the  earth  as  a 
loose  snow-ball  one  or  two  inches  in  diameter.     Sometimes,  after 
the  snow  has  fallen,  it  is  driven  along  by  the  wind,  and  is  rolled 
into  balls  of  vast  size.     These  balls  are  usually  cylindrical,  some- 
what hollowed  in  the  centre,  and  they  have  been  known  to  attain 
a  diameter  of  three  feet.     They  are  of  common  occurrence  on  the 
slopes  of  the  Alps,  in  Switzerland. 

236.  Snow  White  and  Phosphorescent. — Since  snow  is  but  frozen 
water,  it  might  be  expected  that  it  would  be  transparent  like  wa- 
ter, or  large  blocks  of  pure  ice.    Its  brilliant  whiteness  is  due  main- 
ly to  the  number  of  reflecting  surfaces  arising  from  the  small  size 
of  the  spiculae  of  ice.     In  the  same  manner,  the  most  transparent 
glass  loses  its  transparency  when  pulverized. 

Snow  is  feebly  phosphorescent.  This  is  proved  by  the  fact  that 
in  the  darkest  nights,  when  the  ground  is  covered  with  snow,  the 
snow  appears  more  luminous  than  the  sky.  Its  light  can  not. 


120  METEOKOLOQY. 

therefore,  be  simply  the  reflected  light  of  the  sky.  This  phos- 
phorescence appears  to  be  in  part  acquired  by  exposure  to  the 
rays  of  the  sun  during  the  preceding  day.  If,  on  the  morning  of 
a  clear  day,  we  cover  a  portion  of  the  snow  with  an  opaque  screen, 
and  uncover  it  at  evening,  we  shall  find  that  this  portion  is  some- 
what less  luminous  than  the  surrounding  snow.  Snow,  like  many 
other  substances,  after  being  exposed  to  a  bfight  light,  retains  a 
portion  of  the  light  for  some  time  after  the  source  of  light  is  with- 
drawn. 

237.  Red  Snow  in  the  Polar  Regions. — In  those  places  where 
snow  lies  unmelted  from  one  year  to  another,  it  sometimes  ac- 
quires a  ruddy  color,  and  occasionally  becomes  red  like  blood. 
This  occurs  in  the  polar  regions,  and  also  on  the  mountains  of 
Southern  Europe.     In  Spitzbergen  the  snow  sometimes  appears 
of  a  green  hue.     It  has  been  discovered  that  these  colors  are  due 
to  a  vegetable  production  resembling  a  mushroom,  which  is  exces- 
sively minute,  not  exceeding  y-oVoth  inch  in  diameter.     There  is, 
then,  a  species  of  vegetation  which  may  flourish  at  a  temperature 
which  never  exceeds  that  of  melting  ice. 

238.  Glaciers. — The  summits  of  high  mountains,  even  under 
the  equator,  are  covered  with  perpetual  snow.     Within  the  trop- 
ics the  limit  of  perpetual  snow  varies  from  16,000  to  18,000  feet, 
while  on  the  Alps  of  Switzerland  it  varies  from  8000  to  9000  feet. 
On  these  mountains  the  snow  accumulates  from  year  to  year,  and 
in  sheltered  ravines,  where  it  can  not  be  blown  away  by  the  wind, 
acquires  an  immense  thickness.     Under  continued  pressure  this 
snow  becomes  solidified,  so  as  to  acquire  the  density  of  compact 
ice.     The  gorges  of  the  Alps  are  filled  with  ice  of  this  descrip- 
tion, which  is  known  by  the  name  of  glaciers.     These  glaciers  are 
from  five  to  ten  or  more  miles  in  length,  and  they  follow  the 
gorges  from  the  summit  of  Mount  Blanc  down  to  the  base  of  the 
mountain.     They  are  frequently  half  a  mile  or  more  in  breadth, 
and  have  a  thickness  of  200  to  5000  feet.    This  ice,  sustaining  the 
pressure  of  a  long  column,  rising  to  the  height  of  10,000  or  12,000 
feet,  is  crowded  down  into  the  valleys,  so  that  the  entire  glacier 
has  a  descending  motion  like  a  river.     The  principal  glacier  of 
Switzerland  has   a  descending  motion    which    in   some   places 
amounts  to  876  feet  in  a  year,  and  in  other  places  only  274  feet. 


PRECIPITATION   OF   THE   VAPOR   OF   THE   AIR. 


12V 


This  motion  is  continuous,  and  is  probably  never  wholly  inter- 
rupted. Nevertheless,  the  motion  is  greatest  in  summer  and 
least  in  winter,  and  the  velocity  increases  with  the  angle  of  de- 
scent. The  middle  of  the  glacier  generally  moves  faster  than  the 
sides.  These  glaciers  extend  down  into  the  valleys,  where  the 
temperature  is  such  as  to  allow  wheat  and  potatoes  to  come  to 
maturity,  and  a  traveler  may  sometimes  stand  upon  the  edge  of  a 
glacier  and  pick  ripe  cherries  from  a  tree.  The  ice  melts,  indeed, 
under  a  summer's  sun,  but  the  waste  of  summer  is  supplied  by  the 
slow  motion  of  the  descending  mass,  so  that  the  lower  end  of  the 
glacier  remains  nearly  stationary  from  age  to  age.  Fig.  53  rep- 
resents one  of  the  most  remarkable  glaciers  of  the  Alps.  It  is 
seen  to  be  intersected  by  numerous  fissures,  caused  by  its  mo- 
tion down  an  irregular  valley. 


The  total  number  of  glaciers  among  the  Alps  is  estimated  at 
between  500  and  600,  and  they  cover  an  area  of  nearly  1500 
square  miles.  The  lowest  of  the  glaciers  of  the  Alps  descends  to 
the  level  of  3400  feet  above  the  sea. 

239.  Oeograplucal  Distribution  of  Glaciers. — No   glaciers  have 


128  METEOROLOGY. 

been  found  within  the  tropics,  but  they  are  common  on  the  high 
mountains  of  the  middle  latitudes,  and  especially  in  the  polar  re- 
gions. The  glaciers  of  the  Himalayas  are  very  numerous  and  of 
immense  extent,  and  are  the  sources  of  large  rivers.  In  lat.  27° 
they  descend  to  the  level  of  13,000  feet,  and  in  lat.  36°  they  de- 
scend to  the  level  of  9000  feet. 

The  Pyrenees  are  nearly  destitute  of  true  glaciers. 

The  elevated  mountains  of  Greenland  are  covered  with  perpet- 
ual snow  and  ice,  which  in  many  places  extends  to  the  sea-shore. 
The  snow  of  winter  becomes  solidified  by  the  warmth  of  summer, 
acquiring  in  time  the  density  of  ice.  This  ice  is  crowded  down 
by  its  own  weight  into  the  sea,  and  sometimes  extends  several 
miles  beyond  the  original  shore-line.  By  the  buoyant  power  of 
the  water  the  outer  end  of  the  glacier  is  lifted,  and  after  a  time  a 
mass,  perhaps  a  mile  or  more  in  diameter,  is  cracked  off.  This 
mass  is  drifted  southward  to  the  middle  latitudes,  and  is  called 
an  iceberg.  An  iceberg  has  been  measured  three  fourths  of  a 
mile  square,  and  315  feet  high.  Large  icebergs  continue  unmelt- 
ed  for  many  weeks,  and  sometimes  advance  to  lat.  36°. 

In  Norway  the  glaciers  are  numerous,  and  near  lat.  60°  one  of 
them  descends  to  within  150  feet  of  the  sea  level,  while  in  lat.  70° 
they  descend  into  the  sea. 

In  Spitzbergen  one  glacier  presents  a  front  of  eleven  miles  to 
the  sea,  with  a  cliff  400  feet  high,  and  extends  backward  to  the 
mountains. 

The  interior  of  Iceland  is  covered  with  glaciers. 

On  the  west  coast  of  Patagonia  glaciers  are  numerous,  and  in 
lat.  46°  S.  they  descend  to  the  sea. 

The  glaciers  of  Victoria  Land,  lat.  70  to  79°  S.,  are  even  more 
extensive  than  those  of  Greenland. 

240.  Avalanches  of  Snow. — The  snow  which  during  winter  ac- 
cumulates on  the  sides  of  the  Alps  and  other  mountains,  becomes 
softened  during  the  summer,  and  frequently  descends  into  the  val- 
leys in  large  masses  called  avalanches.  During  summer  these 
avalanches  are  of  hourly  occurrence  on  some  parts  of  the  Alps, 
sweeping  down  a  slope  of  several  miles  into  the  valleys,  and  are 
among  the  chief  dangers  encountered  by  travelers  who  attempt 
to  climb  the  mountains. 


PRECIPITATION  OF  THE  VAPOR  OF  THE   AIR.  129 


SECTION  VII. 

HAIL. 

241.  Sleet. — In  the  middle  latitudes,  in  the  cold  months  of  the 
year,  during  gusty  weather,  there  often  fall  from  the  sky  small 
spheres  of  ice,  having  a  diameter  of  one  twelfth  to  one  sixth  of 
an  inch.     They  are  generally  soft,  opaque,  and  of  a  whiteness  ap- 
proaching that  of  snow.     The  largest  are  sometimes  surrounded 
with  a  slight  film  of  ice.     Sometimes  small  hailstones  consist  en- 
tirely of  transparent  ice,  and  these  are  probably  rain-drops  falling 
from  clouds  brought  by  south  winds,  which  freeze  in  traversing 
cold  strata  of  air  near  the  earth. 

The  small  hailstones  of  winter  are  termed  sleet,  to  distinguish 
them  from  large  hail,  which  falls  under  different  circumstances. 

242.  Large  Hail. — 1Large  hail  seldom  if  ever  falls  except  during 
thunder-storms.     It  falls  at  the  commencement  of  the  storm  or 
during  its  continuance.     It  very  rarely  follows  rain,  especially  if 
the  rain  has  continued  for  some  time.     The  area  covered  by  the 
rain-storm  is  much  larger  than  that  covered  by  the  hail,  and  the 
hail  at  any  one  place  continues  but  a  very  short  time,  generally 
only  five  or  ten  minutes,  seldom  so  long  as  fifteen  or  twenty  min- 
utes. 

In  the  United  States  large  hail  falls  chiefly  in  summer  and  the 
latter  part  of  spring.  In  India  hail  falls  chiefly  in  the  four  months 
from  February  to  May. 

Hail  falls  at  all  hours  of  the  day  and  night,  but  large  hail  is 
most  common  about  the  hottest  part  of  the  day,  that  is,  about  2 
P.M.  The  fall  of  large  hail  is  commonly  preceded  by  an  unusual 
degree  of  heat.  An  extraordinary  rise  of  the  thermometer  in 
April  or  May  affords  reason  to  anticipate  a  hail-storm. 

243.  Size  of  Hailstones. — The  size  of  hailstones  varies  from  one 
tenth  of  an  inch  or  less  in  diameter  to  more  than  four  inches.  On 
the  13th  of  August,  1851,  about  1  P.M.,  hailstones  fell  in  New 
Hampshire  weighing  18  ounces.     A  sphere  of  solid  ice  weighing 
18  ounces  has  a  diameter  of  four  inches,  and  a  circumference  of 
12^-  inches.     In  the  present  case  the  stones  were  somewhat  po- 

and  of  irregular  shape,  and  their  largest  circumference  ex- 


130  METEOROLOGY. 

ceeded  15  inches.  A  few  years  since,  hailstones  weighing  sixteen 
ounces  feii  in  the  city  of  Pittsburg,  and  hailstones  weighing  over 
half  a  pound  have  fallen  in  several  places  of  the  United  States. 

On  the  7th  of  May,  1822,  there  fell  at  Bonn,  in  Germany,  hail- 
stones weighing  from  twelve  to  thirteen  ounces,  and  stones  weigh- 
ing half  a  pound  have  repeatedly  fallen  in  France  and  Italy. 

Large  hail  is  of  common  occurrence  in  India.  On  the  llth  of 
May,  1855,  about  6  P.M.,  near  the  Himalaya  Mountains,  in  lati- 
tude 29°,  hailstones  fell  weighing  from  eight  to  ten  ounces,  and 
one  or  two  weighed  more  than  a  pound. 

On  the  22d  of  May,  1851,  in  latitude  13°  north,  in  the  southern 
part  of  India,  many  hailstones  fell  about  the  size  of  oranges.  The 
next  morning,  in  a  dry  well,  there  was  found  a  block  of  ice  meas- 
uring 4-^-  feet  long,  3  feet  broad,  and  18  inches  thick.  It  is  not 
supposed  that  this  ice  fell  from  the  sky  in  a  single  block,  but 
after  their  fall  the  separate  hailstones  became  cemented  together 
so  firmly  by  ice  as  to  form  one  solid  block.  Similar  masses  of 
ice  derived  from  hail  have  been  repeatedly  seen  in  India,  and 
also  in  the  United  States. 

244.  Quantity  of  Hail. — The  quantity  of  hail  which  falls  from 
the  sky  in  a  single  shower  is  sometimes  enormous.     In  the  New 
Hampshire  storm  of  1851  the  average  depth  of  the  hail  was  four 
inches.     In  a  storm  which  passed  over  the  Orkneys,  on  the  north 
of  Scotland,  July  24th,  1818,  the  depth  of  the  hail  was  nine  inches. 
On  the  17th  of  August,  1830,  in  the  streets  of  Mexico,  hail  fell  to 
the  depth  of  sixteen  inches. 

245.  Form  of  Hailstones. — Hailstones  are  ordinarily  of  a  sphe- 
roidal form;  sometimes  they  are  oval,  sometimes  flattened,  and 
sometimes  of  a  very  irregular  shape.    Very  large  hailstones  often 
present  remarkable  protuberances.     They  often  consist  of  an  ir- 
regular assemblage  of  angular  pieces  of  ice,  which  individually 
do  not  exceed  the  size  of  walnuts,  but  cemented  firmly  together, 
forming  a  mass  as  large  as  an  orange,  and  sometimes  as  large  as 
a  turkey's  egg.     These  small  portions  generally  indicate  a  .tend- 
ency to  crystallization.     Sometimes  hailstones  are  studded  with 
crystals  in  the  form  of  hexagonal  prisms,  and  when  the  angles  melt 
away  the  prisms  become  nearly  cylindrical.    The  following  fig- 
ure represents  a  hailstone  which  probably  consisted  originally 


PRECIPITATION   OF  THE  VAPOR   OF  THE   AIR. 


131 


- 54-  of  numerous  prisms  ce- 

mented together,  but  it 
became  so  modified  by 
melting  during  its  fall  as 
nearly  to  obliterate  the 
crystalline  structure. 

Sometimes  hailstones 
have  the  form  of  pyra- 
mids, whose  angles  are 
rounded  by  a  partial 
melting,  and  whose  base 
is  a  portion  of  an  irregular  spherical  surface. 

246.  Structure  of  Hailstones.  —  The  centre  of  large  hailstones 
usually  consists  of  hardened  snow,  and  this  is  surrounded  by  a 
coat  of  transparent  ice.     Sometimes  we  find  alternate  layers  of 

opaque  snow  and  transparent  ice.  Often 
hailstones  exhibit  a  radiated  structure,  re- 
sulting apparently  from  rows  of  air-bubbles 
disposed  in  radii  from  the  centre.  Some- 
times large  hailstones  consist  of  very  trans- 
parent and  solid  ice  with  numerous  air-bub- 
bles. Fig.  56  represents  a  section  of  a  hail- 
stone whose  external  appearance  is  repre- 
sented in  Fig.  55.  Hailstones  with  a  radi- 
ated structure,  when  broken,  incline  to  di- 
vide into  spherical  pyramids,  with  layers 
parallel  to  their  base,  and  this  is  probably 
the  origin  of  pyramidal  hailstones.  The 
rupture  of  the  spherical  hailstone  may  be 
due  to  the  sudden  expansion  experienced 

in  passing  from  an  exceedingly  cold  to  a  comparatively  warm. 

atmosphere. 

247.  Geographical  Distribution  of  Hail. — Within  the  tropics  hail 
is  of  rare  occurrence  at  the  level  of  the  sea,  but  when  it  does  oc- 
cur the  stones  are  generally  of  very  great  size.     It  becomes  more 
common  at  the  height  of  1500  feet.     In  India,  hail  is  very  com- 
mon on  the  mountains,  and  occurs  occasionally  at  the  level  of  the 
sea,  even  south  of  latitude  20°. 


132 


METEOROLOGY. 


Hail  is  most  common  in  the  middle  latitudes.  In  Europe  hail 
occurs  most  frequently  near  the  Atlantic  coast,  and  diminishes  in 
frequency  as  we  proceed  eastward.  In  France  hail  falls,  on  an 
average,  fifteen  times  in  a  year  ;  in  Germany,  five  times ;  and  in 
Kussia  only  three  times.  Hail  falls  in  every  part  of  the  United 
States,  but  cases  of  very  large  hail  occur  but  seldom.  Hail  falls 
occasionally,  but  not  often,  in  the  West  India  Islands. 

248.  Track  of  Hail-storms. — Hail-storms  usually  travel  rapidly 
over  the  country  in  straight  bands  of  small  breadth,  but  consid- 
erable length.     The  track  of  the  New  Hampshire  storm  was  sev- 
eral miles  in  length,  but  only  two  miles  in  breadth.     The  track 
of  the  Orkney  storm  was  twenty  miles  long  and  a  mile  and 
a  half  wide,  and  the  storm  traveled  at  the  rate  of  forty  miles  per 
hour. 

On  the  13th  of  July,  1788,  a  hail-storm  traveled  from  the  S.W. 
part  of  France  to  the  shores  of  Holland  at  the  rate  of  46  miles 
per  hour.  There  were  two  distinct  bands  of  hail,  the  breadth  of 
that  in  the  west  being  eleven  miles,  and  that  in  the  east  six  miles, 
with  a  space  of  fourteen  miles  between  them.  The  fall  of  hail 
upon  these  two  bands  was  not  exactly  contemporaneous,  but  one 
preceded  the  other  about  fifteen  minutes.  Rain  fell  on  the  outside 

of  these  bauds  of  hail, 
as  well  as  on  the  space 
between  them.  Each 
band  of  hail  extended  a 
distance  of  about  500 
miles.  Figure  57  repre- 
sents a  portion  of  the 
track  of  this  storm  in 
the  neighborhood  of 
Paris.  The  dotted  bands 
represent  the  track  of 
the  hail,  while  the  three 
shaded  bands  represent 
the  area  of  the  rain. 

249.  Height  at  which  Hail  is  formed.  —  Observations  made  in 
mountainous  countries  have  enabled  us  to  determine  nearly  the 
elevation  at  which  hail  is  formed.     Small  hail  is  of  common  oc- 


Fig.  5T. 


PRECIPITATION   OF  THE  VAPOR  OF  THE   AIR.  133 

currence  on  the  summit  of  Mount  Blanc,  15,744  feet  above  the 
level  of  the  sea,  but  large  hail  has  never  been  seen  there.  In  In- 
dia, at  the  height  of  8000  feet,  hailstones  have  fallen  of  sufficient 
size  to  do  considerable  damage. 

In  1835,  hailstones  weighing  eight  ounces  fell  at  the  base  of  a 
mountain  in  the  southern  part  of  France,  while  only  small  hail 
fell  at  the  height  of  4000  to  5000  feet.  From  these  and  similar 
observations,  it  is  inferred  that,  in  the  middle  latitudes,  hail  often 
begins  to  form  at  an  elevation  exceeding  16,000  feet,  but  attains 
its  greatest  size  below  the  height  of  5000  feet. 

250.  Origin  of  the  Cold  which  causes  Hail.  —  The  cold  which 
congeals  such  large  masses  of  ice  in  summer  is  mainly  due  to  ele- 
vation.    The  temperature  of  hailstones  at  the  instant  of  their  fall 
has  often  been  found  below  32°,  and  sometimes  as  low  as  25°  F. 
They  must,  then,  have  been  subjected  to  a  temperature  considera- 
bly below  that  of  melting  ice,  probably  to  a  temperature  as  low 
as  20°  F.     In  the  neighborhood  of  New  York,  at  the  height  of 
18,000  feet,  the  average  summer  temperature  is  20°,  and  it  is  be- 
lieved that  during  the  formation  of  hail  the  temperature  of  the 
upper  air  is  considerably  below  the  mean. 

251.  Noise  preceding  the  Fall  of  Hail. — Some  seconds  before  the 
fall  of  hail,  and  occasionally  several  minutes,  a  peculiar  crackling 
noise  is  often  heard  in  the  air.     It  has  been  compared  to  the 
noise  of  walnuts  violently  shaken  up  in  a  bag.     This  noise  has 
been  ascribed  to  the  great  velocity  with  which  the  hailstones  are 
driven  through  the  air,  while  some  have  ascribed  it  to  feeble  elec- 
trical discharges  from  one  hailstone  to  another,  for  electricity  al- 
ways attends  the  progress  of  a  hail-storm. 

252.  Hail  attended  by  Two  Currents  of  Air. — The  formation  of 
hail  is  invariably  attended  by  two  distinct  currents  of  air,  and  one 
of  these  currents  displaces  the  other  with  great  violence.     The 
current  of  air  which  precedes  the  approach  of  a  hail-storm  is  ex- 
tremely hot,  and  highly  charged  with  moisture;  that  which  suc- 
ceeds the  fall  of  hail  has  an  icy  chillness.     The  warm  and  humid 
air  is  displaced  by  the  cold  current,  and  is  thus  forced  up  to  a 
great  elevation  above  the  earth,  by  which  means  its  vapor  is  sud- 
denly condensed.    Upon  the  front  of  the  hail-cloud  this  condensed 


134  METEOROLOGY. 

vapor  exists  in  the  form  of  water,  whose  temperature  is  near  32°. 
In  the  interior  of  the  hail-cloud  the  vapor  is  precipitated  in  the 
form  of  snow,  whose  temperature  is  sometimes  as  low  as  20°. 

253.  Process  of  the  formation  of  Hail.  —  Observations  on  the 
summits  of  mountains  have  shown  that,  on  the  front  of  the  hail- 
cloud,  there  exists  a  violent  whirling  motion  about  a  horizontal 
axis.     This  whirling  motion  causes  the  snow  to  collect  in  small 
balls,  each  of  which  forms  the  nucleus  of  a  hailstone.    The  snow- 
ball is  forced  into  the  warm  current,  where  it  receives  a  layer  of 
water,  which  is  congealed  by  the  cold  of  the  nucleus,  thus  render- 
ing the  snowy  centre  more  compact,  and  adding  a  shell  of  trans- 
parent ice.     By  means  of  the  whirling  motion,  the  hailstone,  cov- 
ered with  a  stratum  of  uucongealed  water,  is  hurled  into  the  snow- 
cloud,  where  it  receives  a  layer  of  snow,  and  again  becomes  thor- 
oughly chilled.    Thence  it  escapes  again  into  the  water-cloud,  and 
is  covered  with  a  layer  of  water,  which  is  congealed  by  the  cold 
of  the  nucleus.     Thus,  by  the  whirling  motion,  it  is  plunged  al- 
ternately into  the  snow-cloud  and  the  water-cloud,  while  each  al- 
ternation furnishes  a  layer  of  spongy  ice  and  a  layer  of  transpa- 
rent ice.     Thus  the  stone  grows  with  immense  rapidity,  and  in  a 
few  minutes  becomes  a  large  ball,  three  or  four  inches  in  diame- 
ter.   This  oscillatory  motion  of  the  hailstones,  on  the  front  of  the 
hail-cloud,  was  distinctly  observed  by  M.  Lecoq  in  1835  on  the 
summit  of  a  mountain  in  the  southern  part  of  France. 

254.  How  Hail  is  sustained  in  the  Air. — The  hailstones  are  sus- 
tained in  the  air  by  the  violent  upward  motion  caused  by  the  cold 
current  displacing  the  warm  one.     A  sphere  of  ice  two  inches  in 
diameter,  by  falling  through  a  tranquil  atmosphere,  soon  acquires 
a  velocity  of  90  feet  per  second.     A  hailstone  of  irregular  shape 
would  experience  more  resistance  than  a  sphere,  and  would  ac- 
quire a  somewhat  less  velocity,  but  it  would  still  fall  from  a  height 
of  18,000  feet  in  about  three  minutes,  which  time  is  too  small  to 
allow  the  formation  of  masses  of  ice  weighing  one  pound.     An 
upward  current  of  air  rising  with  a  velocity  of  90  feet  per  second 
would  sustain  a  sphere  of  ice  two  inches  in  diameter,  and  would 
greatly  reduce  the  velocity  of  stones  of  larger  size. 

255.  How  long  may  Hailstones  be  sustained? — The  strong  up* 


PRECIPITATION  OF  THE   VAPOR  OF  THE  AIR.  135 

ward  movement  which  always  attends  the  formation  of  hail  is 
probably  sufficient  to  sustain  hailstones  of  the  largest  size  as  long 
as  they  can  be  kept  within  the  influence  of  this  vortex.  A  period 
of  ten  minutes  is  probably  sufficient  for  the  formation  of  hail- 
stones of  the  largest  size.  After  escaping  from  the  influence  of 
this  vortex,  small  stones  would  fall  to  the  earth,  from  an  eleva- 
tion of  5000  feet,  in  about  two  minutes,  and  very  large  stones  in 
one  minute. 

256.  Origin  of  the  Parallel  Bands  of  Hail. — It  is  not  uncommon 
for  two  or  even  three  such  vortices  to  form  on  the  same  day,  and 
nearly  at  the  same  hour,  at  places  not  very  remote  from  each  oth- 
er, thus  forming  parallel  bands  of  hail  separated  by  an  interval 
of  from  10  to  100  miles.     Such  was  the  French  storm  of  1788, 
and  similar  cases  have  frequently  occurred  in  the  United  States. 

257.  Origin  of  Sleet. — The  small,  spongy  hail  of  winter  is  prob- 
ably formed  in  the  same  way  as  the  large  hail  of  summer;  but 
since,  in  winter,  the  amount  of  vapor  present  in  the  air  is  small, 
the  amount  of  precipitation  is  small,  and  the  hailstones  can  never 
attain  a  large  size. 

258.  Hail-rods. —  It  has  been  proposed  to  preserve  vineyards 
and  valuable  farms  from  the  ravages  of  hail  by  erecting  an  im- 
mense number  of  poles,  armed  with  iron  points,  communicating 
with  the  earth,  for  the  purpose  of  drawing  off  the  electricity  of 
the  clouds.     Multitudes  of  these  hail-rods  were  formerly  erected 
in  Switzerland,  but  without  the  expected  success. 

It  is  believed  that  electricity  performs  altogether  a  subordinate, 
if  not  an  unimportant  part  in  the  formation  of  hail ;  and  if  we 
could  draw  off  all  the  electricity  from  the  hail-cloud  as  fast  as  it 
was  generated,  it  is  not  improbable  that  hail  would  be  formed 
about  as  large  and  as  abundantly  as  at  present. 

But,  even  supposing  electricity  to  be  the  sole  agent  in  the  pro- 
duction of  hail,  hail-rods  could  not  be  expected  to  furnish  security 
against  hail  unless  an  entire  continent  could  be  studded  thick 
with  them,  for  in  the  middle  latitudes  the  hail-cloud  advances 
eastward  with  a  velocity  sometimes  of  40  or  more  miles  per  hour, 
and  the  hailstones  which  fall  in  one  locality  are  those  which  were 
forming  when  the  cloud  was  many  miles  westward  of  that  point, 


136  METEOROLOGY. 

so  that,  to  protect  a  small  spot,  the  whole  country  for  many  miles 
westward  should  be  armed  with  rods  ;  and  it  is  conceivable  that 
a  hail-cloud  arriving  over  a  region  studded  with  these  rods  might 
immediately  pour  down  a  large  quantity  of  hailstones  which 
would  have  fallen  farther  eastward  if  the  rods  had  not  discharged 
the  electricity  of  the  cloud. 


CHAPTER  VI 

STORMS,  TORNADOES,  AND  WATER-SPOUTS. 

SECTION  I. 

THEORY  AND   LAWS    OP  STORMS. 

259.  What  is  a  Storm  f — Any  violent  and  extensive  commotion 
of  the  atmosphere  is  called  a  storm.     Storms  are  usually  attended 
by  a  fall  of  rain,  or  snow,  or  hail,  and  frequently  by  thunder  and 
lightning;  but,  although  it  is  probable  that  a  precipitation  of  va- 
por always  takes  place  over  some  portion  of  the  area  of  every  vio- 
lent storm,  yet  the  storm  often  extends  beyond  the  area  of  rain  or 
snow. 

260.  Cause  of  Storms. — Storms  are  caused  by  a  strong  and  ex- 
tensive upward  motion  of  the  air,  by  which  means  its  vapor  is 
condensed  by  the  cold  of  elevation. 

The  atmosphere  receives  heat  from  the  sun,  and  it  loses  heat 
by  radiation.  Only  about  one  fourth  of  the  rays  of  the  sun  are 
absorbed  in  passing  vertically  through  the  atmosphere.  The  re- 
maining three  fourths  are  absorbed  by  the  earth's  surface,  by 
which  means  its  temperature  is  raised,  and  heat  is  thence  commu- 
nicated to  the  air  which  rests  upon  the  earth.  The  lower  strata 
of  the  atmosphere  thus  become  rapidly  heated,  while  the  upper 
strata  acquire  heat  more  slowly. 

Since  the  density  of  the  air  is  diminished  by  an  increase  of 
heat,  the  atmosphere  is  in  a  state  of  unstable  equilibrium,  and  the 
lower  strata  tend  continually  to  rise  and  take  the  place  of  the  up- 
per. Such  ascending  currents  are  formed  on  every  tranquil  day. 
As  the  air  ascends  it  comes  under  diminished  pressure  and  ex- 
pands, and  as  it  expands  it  cools  at  the  rato  of  about  38  degrees 


STORMS,   TORNADOES,   AND  WATER-SPOUTS.  137 

for  two  miles  of  ascent  This  ascending  air  carries  with  it  the 
vapor  which  it  contained  at  the  earth's  surface,  and,  if  it  rises 
high  enough,  the  cold  produced  by  expansion  will  condense  a 
portion  of  this  vapor  into  cloud.  The  height  to  which  the  air 
must  ascend  before  it  will  become  cold  enough  to  form  cloud  de- 
pends upon  the  difference  between  the  dew-point  and  the  tem- 
perature of  the  air.  If  the  dew-point  be  ten  degrees  below  the 
temperature  of  the  air,  cloud  will  begin  to  form  when  the  ascend- 
ing current  has  risen  about  3000  feet. 

261.  Latent  Heat  liberated. — As  soon  as  a  cloud  begins  to  form, 
the  latent  heat  of  the  vapor  is  liberated.     To  convert  water  into 
vapor  requires  a  great  amount  of  heat,  and  this  heat  is  not  appre- 
ciable to  a  thermometer;  hence  it  is  called  latent  heat.     When 
the  vapor  returns  to  the  condition  of  water,  this  heat  is  liberated, 
and  becomes   sensible  heat;  and  when  a  cubic  foot  of  water  is 
precipitated  from  the  air,  as  much  heat  is  liberated  as  would  be 
required  to  convert  that  amount  of  water  into  vapor.    When  one 
inch  of  rain  falls  from  the  sky,  the  amount  of  water  precipitated 
exceeds  two  millions  of  cubic  feet  for  each  square  mile  of  surface; 
and  over  each  square  mile  of  surface  as  much  heat  would  be  lib- 
erated as  would  be  required  to  evaporate  two  millions  of  cubic 
feet  of  water. 

By  the  heat  thus  liberated  in  the  production  of  a  cloud,  the  air 
in  the  cloud  is  warmed;  it  expands  in  volume,  and  the  cloud  con- 
tinues to  ascend  as  long  as  its  temperature  is  greater  than  that  of 
the  surrounding  air.  As  the  cloud  ascends,  more  vnpor  is  con- 
densed, while  the  latent  heat  evolved  raises  still  farther  the  tem- 
perature of  the  air  in  the  cloud. 

262.  Shape  of  the  Cloud  thus  formed. — When,  in  consequence 
of  an  ascending  column  of  air,  a  cloud  begins  to  form,  it  is  seen  to 
swell  out  at  the  top,  but  its  base  continues  at  the  same  level ; 
that  is,  the  base  is  flat,  even  after  the  cloud  has  acquired  great 
vertical  height     The  motion  of  the  air  here  described  is  illus- 
trated by  Fig.  58,  on  the  following  page.     During  a  warm  and 
tranquil  day  many  of  these  ascending  columns  are  formed,  and 
two  or  more  adjacent  columns  often  unite  to  form  a  single  col- 
umn. 

The  clouds  thus  formed  during  the  day  often  subside  and 


138 


METEOROLOGY. 


Pig.  68. 


dissolve  at  evening,  when  the 
surface  of  the  earth  becomes 
cooled  by  radiation,  and  thus  a 
cloudy  day  is  often  followed 
by  a  cloudless  evening;  but 
when  the  atmosphere  is  unu- 
sually heated,  and  contains  a 
large  amount  of  vapor,  the  as- 
cending columns  generally  go 
on  increasing  until  rain  de- 
scends. 


263.  Effect  of  the  Earttis  Rotation. — When  from  any  cause  the 
air  begins  to  flow  from  all  directions  toward  a  central  region, 
the  direction  of  its  motion  is  affected  by  the  earth's  rotation  on 
its   axis.     As   in  the   experiment  with  a  free   pendulum   (see 
Loomis'  Astronomy,  Art.  51),  every  current  in  the  northern  hem- 
isphere by  the  rotation  of  the  earth  is  deflected  to  the  right  of 
what  would  otherwise  have  been  its  course,  and  this  deflection 
causes  a  diminution  of  pressure  on  the  left  side  of  the  current. 
When  water  has  a  movement  of  rotation  in  a  stationary  basin, 
the  centrifugal  force  produced  by  the  rotation  causes  the  water 
to  recede  from  the  centre,  and  the  surface  rises  toward  the  edge 
of  the  vessel,  so  that  the  pressure  on  the  bottom  of  the  vessel  di- 
minishes from  the  circumference  to  the  centre.     When  currents 
of  air  flow  from  all  directions  toward  a  central  area,  a  similar 
effect  is  produced  by  the  earth's  rotation,  so  that  there  is  a  gradual 
diminution  of  pressure  from  the  circumference  of  this  area  to  the 
centre.     The  air  thus  moves  inward  by  a  curvilinear  path,  and 
the  resulting  centrifugal  force  diminishes  still  further  the  pressure 
upon  the  earth's  surface.     The  amount  of  the  effect  produced  by 
the  earth's  rotation  depends  upon  the  latitude  of  the  place.     It  is 
zero  at  the  equator,  and  it  increases  as  the  sine  of  the  latitude. 

264.  Barometric  Gradient. — The  diminution  of  pressure  toward 
the  centre  of  a  revolving  mass  of  air  is  conveniently  indicated  by 
the  change  in  the  height  of  the  barometer  in  the  distance  of  100 
miles,  and  this  change  is  called  the  barometric  gradient.     In  a 
given  latitude,  the  gradient  depends  upon  the  velocity  of  the 


STORMS,  TORNADOES,  AND  WATER-SPOUTS.  139 

wind,  and  upon  the  distance  from  the  centre  of  the  revolving 
mass  of  air.  In  latitude  40°,  with  a  wind  blowing  30  miles  per 
hour,  at  a  distance  of  100  miles  from  the  centre  of  the  revolving 
mass,  according  to  theory  the  gradient  should  be  0.16  inch  in  100 
miles,  and  at  a  distance  of  400  miles  from  the  centre  the  gradient 
should  be  0.10  inch  in  100  miles;  so  that  when  the  air  is  set  in 
motion  with  a  velocity  of  30  miles  per  hour  over  a  region  several 
hundred  miles  in  diameter,  the  gradient  gradually  increases  from 
the  circumference  to  the  centre,  and  the  total  depression  of  the 
barometer  at  the  centre  may  be  an  inch  below  its  normal  height 
If  the  wind  has  a  greater  velocity,  the  depression  at  the  centre 
may  amount  to  two  inches,  and  even  more. 

265.  Depression  of  the  Barometer  in  Storms. — Storms  are  accom- 
panied by  a  considerable  depression  of  the  barometer  below  its 
mean  height,  and  are  generally  preceded  as  well  as  followed  by  a 
pressure  greater  than  the  mean.     During  the  passage  of  a  winter 
storm  over  the  middle  latitudes  of  North  America,  the  barometer 
sometimes  sinks  below  its  mean  height  over  an  area  more  than 
a  thousand  miles  in  diameter,  and  the  depression  at  the  centre  is 
sometimes  more  than  an  inch  below  the  mean.    Upon  the  Atlan- 
tic Ocean,  the  barometer  has  been  known  to  fall  below  its  mean 
height  over  an  area  more  than  two  thousand  miles  in  diameter, 
and  the  depression  at  the  centre  has  been  known  to  be  two  and 
a  half  inches  below  its  mean  height. 

266.  Form  of  the  Isobaric  Curves. — The  lines  of  equal  barometric 
pressure  (called  isobaric  curves,  or  simply  isobars)  surrounding 
the  centre  of  a  great  storm  are  generally  of  an  irregular  oval  form. 
Occasionally  they  differ  but  little  from  circles;  but  more  frequent- 
ly they  are  considerably  elongated.     Generally  the  mnjor  axis  of 
the  isobars  exceeds  the  minor  axis  by  one  half  its  whole  length ; 
and  occasionally  the  longest  diameter  is  four  or  five  times  the  least 

267.  Areas  of  High  Barometer. — While  around  every  storm 
centre  there  is  a  considerable  area  of  low  barometer,  beyond  the 
borders  of  the  storm  there  are  extensive  regions  where  the  barom- 
eter is  above  its  mean  height.     These  areas  of  high  barometer 
have  an  irregular  oval  form,  and  undergo  frequent  changes  of 
form  and  position.     Near  the  centre  of  one  of  these  areas  the  ba- 


140  METEOROLOGY. 

rometer  frequently  stands  a  half  inch  above  its  mean  height,  and 
occasionally  an  entire  inch. 

268.  Course  and  Velocity  of  Storms. — The  average  direction  of 
storm  paths  across  the  United  States  is  toward  a  point  a  little 
north  of  east,  but  it  varies  somewhat  with  the  season  of  the  year, 
being  almost  exactly  east  in  summer,  and  inclining  more  to  the 
north  in  winter.    Occasionally  storms  travel  toward  the  southeast, 
and  sometimes  toward  the  northeast;  and  in  a  few  instances  they 
have  been  known  to  advance  for  a  day  or  two  toward  the  north- 
west or  the  southwest.     Their  average  velocity  of  progress  is  26 
miles  per  hour,  being  21  miles  in  summer  and  30  miles  in  winter; 
but  occasionally  they  attain  a  velocity  of  more  than  50  miles  per 
hour,  and  sometimes  they  remain  sensibly  stationary  for  a  day 
or  two.     Areas  of  high  barometer  generally  advance  eastward  at 
about  the  same  rate  as  areas  of  low  barometer,  but  their  course 
ordinarily  inclines  somewhat  toward  the  south  of  east. 

269.  Whence  arises  the  Power  which  sustains  the  Energy  of  a 
Storm? — In  Art.  140  it  was  stated  that  the  wind  blows  from 
places  where  the  barometer  is  highest  toward  places  where  it  is 
most  depressed.     The  low  pressure  near  the  centre  of  a  great 
storm  may  therefore  be  regarded  as  the  cause  of  the  winds  which 
flow  inward  from  all  directions.     But  if  no  other  force  acted,  this 
inflowing  air  would  soon  establish  an  equilibrium  of  pressure, 
and  then  the  winds  would  cease.     On  the  contrary,  this  inward 
movement  of  the  wind  which  always  characterizes  great  storms, 
continues  with  extreme  violence  day  after  day,  and  sometimes 
for  weeks  in  succession.    There  must  then  be  a  sustaining  power 
which  perpetually  recruits  these  violent  winds.     This  power  re- 
sults from  the  precipitation  of  the  vapor  of  the  atmosphere,  and 
the  consequent  liberation  of  a  vast  amount  of  latent  heat.     Vio- 
lent storms  are  always  accompanied  by  a  great  fall  of  rain  or  snow, 
and  the  storm  subsides  as  soon  as  the  rain  or  snow  ceases  to  fall. 
The  heat  liberated  in  the  condensation  of  the  vapor  of  the  air, 
is  therefore  to  be  regarded  as  the  cause  of  the  strong  winds  which 
constitute  violent  storms.     When  vapor  is  precipitated  in  the 
form  of  snow,  the  latent  heat  which  is  liberated  is  one  seventh 
more  than  when  the  precipitation  is  in  the  form  of  water,  and 
its  effect  on  the  force  of  the  winds  is  proportionally  great 


STORMS,  TORNADOES,  AND  WATER-SPOUTS. 


141 


270.  Depression  of  the  Barometer  explained. — The  winds  which 
flow  inward  toward  the  rain  area  are  deflected  to  the  right  in  con- 
sequence of  the  rotation  of  the  earth,  and  hence  results  a  diminu- 
tion of  pressure  on  the  left  side.     The  winds  move  spirally  in- 
ward, circulating  around  the  centre  of  the  rain  area,  and  this  cir- 
culation generates  a  centrifugal  force  which  tends  still  further  to 
depress  the  barometer.     There  are  then  two  distinct  causes  for 
the  depression  of  the  barometer  near  the  centre  of  a  violent  storm, 
viz.,  the  deflection  of  the  winds  produced  by  the  earth's  rotation, 
and  the  centrifugal  force  resulting  from  the  circulation  of  the 
winds  around  the  rain  area.     The  efficiency  of  the  first  cause  is 
proportional  to  the  velocity  of  the  wind;  but  the  centrifugal  force 
varies  as  the  square  of  the  velocity.    In  tropical  cyclones,  the  ve- 
locity of  the  winds  is  sometimes  so  great  that  the  fall  of  the  barom- 
eter is  due  mainly  to  the  second  cause,  and  the  influence  of  the 
first  cause  is  well-nigh  inappreciable ;  but  in  the  ordinary  storms 
of  the  middle  latitudes,  the  first-named  cause  is  the  most  important, 
and  the  influence  of  the  second  cause  is  comparatively  small. 

271.  Theory  Illustrated  by  an  Example. — The  observed  direc. 
tion  of  the  wind  is  greatly  influenced  by  the  inequalities  of  the 
earth's  surface,  as  well 

as  by  local  differences 
of  temperature  and 
moisture ;  neverthe- 
less, in  all  violent 
storms  we  find  abun- 
dant evidence  that  the 
winds  tend  inward, 
and  also  tend  to  cir- 
culate about  the  cen- 
tre of  low  pressure  in 
a  direction  contrary  to 
the  motion  of  the 
hands  of  a  watch. 
Fig.  59  represents  the 
winds  as  observed 
near  the  centre  of  a 
violent  storm  of  rain 
and  snow  which  pre- 


Fig.  59. 


\ 


142  METEOROLOGY. 

vailed  in  the  neighborhood  of  New  York  on  the  16th  of  February, 
1842.  The  smaller  oval  line  shows  the  area  within  which  the 
barometer  fell  eight  tenths  of  an  inch  below  the  mean,  and  the 
larger  oval  shows  the  area  of  seven  tenths  inch  barometric  depres- 
sion. The  long  arrow  represents  the  direction  in  which  the  storm 
advanced.  The  short  arrows  show  the  directions  of  the  wind 
over  an  area  about  500  miles  in  diameter.  Since  the  southeast 
portion  of  this  storm  .extended  over  the  ocean,  there  is  a  deficiency 
of  observations  from  that  side,  and  there  are  some  anomalies  in 
the  observed  directions  of  the  wind  which  may  be  ascribed  to 
the  fact  that  the  observations  were  not  all  made  at  the  same 
hour;  nevertheless,  the  inward  tendency  of  the  winds,  and  the 
circulation  about  the  low  centre  are  distinctly  indicated. 

272.  Additional  Examples. — Fig.  60  represents  the  winds  as 

observed  near  the  centre 

Fig.  60.  . 

Brussels**    f  °*  a  violent  storm  of  ram 

and  snow  which  was  ex- 
perienced  in  Europe  Dec. 
25,  1836.  The  smaller 
oval  shows  the  area  with- 
in which  the  barometer 
was  depressed  three  fourths 
of  an  inch  below  the  mean, 
and  the  larger  oval  shows 
the  area  of  one  half  inch 
barometric  depression. 
The  arrows  show  the  ob- 
served directions  of  the 
wind  over  an  area  about 
900  miles  in  diameter. 

_  SOinch  i 

Here  the  inward  motion 

of  the  winds  is  distinctly  shown,  as  well  as  the  circulation  from 
right  to  left,  and  there  are  no  considerable  anomalies  observed  in 
the  reported  directions  of  the  wind. 

The  principles  already  stated  are  more  fully  illustrated  by  Plate 
III.,  which  represents  some  of  the  phenomena  of  a  remarkable 
storm  which  prevailed  over  the  Atlantic  Ocean  Feb.  5,  1870. 
At  the  centre  of  the  storm  the  barometer  fell  to  27.33  inches, 
and  the  plate  represents  the  isobars,  at  intervals  of  two  tenths  of 


STORMS,  TORNADOES,  AND  WATER-SPOUTS.  143 

an  inch,  from  27.4  inches  to  30.0  inches.  Adjacent  to  the  low 
area  was  an  area  of  high  barometer,  at  the  centre  of  which  the 
pressure  was  31.09  inches.  The  arrows  show  the  direction  and 
force  of  the  wind,  the  force  being  indicated  by  the  number  of 
feathers  on  the  tail  of  each  arrow,  according  to  a  scale  of  1  to  6, 
in  which  1  indicates  a  feeble  wind,  and  6  indicates  a  hurricane. 

It  will  be  seen  that  within  the  low  area  the  winds  are  very 
violent,  especially  near  the  centre,  and  that  they  tend  inward, 
circulating  about  the  low  centre  in  a  direction  contrary  to  the 
motion  of  the  hands  of  a  watch.  Within  the  high  area  the  winds 
are  feeble  and  tend  outwards,  circulating  about  the  high  centre 
in  the  same  direction  as  that  of  the  hands  of  a  watch.  From  the 
centre  of  the  low  area  to  the  centre  of  the  high  area  the  distance 
is  2250  miles,  and  the  difference  in  the  heights  of  the  barometers 
'at  these  two  points  is  3. 76  inches,  which  is  one  eighth  of  the 
mean  pressure  of  the  atmosphere. 

The  diameter  of  this  storm  from  west  to  east  (measured  up  to 
the  isobar  of  30  inches)  was  2250  miles,  and  its  diameter  from 
north  to  south  was  about  2800  miles.  This  was  one  of  the  most 
violent  storms  ever  known  on  the  Atlantic  Ocean ;  but  storms 
of  destructive  violence,  and  having  a  diameter  of  about  2000 
miles,  occur  every  winter  over  this  ocean. 

273.  Distinction  between  the  Direction  of  the  Wind  and  that  of  the 
Storm's  Progress. — It  will  thus  be  seen  that  the  direction  of  the 
wind  at  any  place  is  entirely  distinct  from  that  of  the  storm's 
progress  over  the  earth's  surface.  While  the  storm  advances 
steadily  eastward,  the  wind  has  every  possible  direction  at  differ- 
ent places  within  the  limits  of  the  storm. 

At  places  on  the  north  side  of  the  centre  of  a  great  storm  the 
wind  generally  sets  in  from  the  north  of  east  as  the  storm  ap- 
proaches, and -as  the  storm  passes  by  the  wind  changes  to  the 
northwest,  veering  round  by  the  north  point  At  places  on  the 
south  side  of  the  centre  of  the  storm  the  wind  generally  sets  in 
from  the  south  of  east  as  the  storm  approaches,  and  as  the  storm 
passes  by  the  wind  changes  to  the  southwest,  veering  round  by 
the  south  point. 

Frequently  the  centre  of  a  great  winter  storm  is  situated  be- 
yond the  limits  of  the  United  States  on  the  north,  and  then, 
throughout  the  entire  United  States,  as  far  as  observations  have 


144  METEOROLOGY. 

extended,  the  wind  blows  from  the  E.  or  S.E.  on  the  front  of  the 
storm,  and  from  the  W.  or  S.W.  on  the  rear  of  the  storm. 

274.  Lull  at  the  Centre  of  a  Storm. — Near  the  centre  of  a  great 
storm  there  is  generally  a  lull  of  the  wind,  and  sometimes  a  calm. 
Sometimes  the  clouds  open,  exhibiting  considerable  clear  sky,  and 
occasionally  the  clouds  disappear  entirely  for  several  hours,  ex- 
hibiting a  clear  sky,  with  little  wind  and  a  mild  temperature. 
Soon  after  the  centre  of  the  storm  has  passed  eastward  of  the  ob- 
server the  wind  generally  changes  to  the  west,  and  the  barometer 
begins  to  rise.     The  rain  or  snow,  which  may  have  been  tempo- 
rarily suspended,  is  renewed,  generally  with  considerable  violence, 
which,  however,  in  such  cases,  is  not  usually  of  long  continuance. 

275.  Wind  within  an  Area  of  High  Barometer. — Within  an  area 
of  high  barometer  which  often  prevails  a  little  beyond  the  limits 
of  a  violent  storm,  the  winds  are  generally  feeble,  and  these  winds 
tend  outward  from  the  region  of  greatest  pressure.     Hence  it  re- 
sults that  a  little  beyond  the  limits  of  a  storm  the  winds  blow  in 
nearly  opposite  directions ;  on  the  nearest  side  of  the  area  of  high 
barometer  the  wind  blows  toward  the  storm-centre,  while  on  the 
remoter  side  of  this  area  the  wind  blows  jfrom  the  storm-centre. 

276.  How  Winds  are  Propagated  from  Place  to  Place. — Since  on 
the  opposite  sides  of  a  storm  the  wind  blows  in  nearly  opposite 
directions,  while  the  entire  storm  makes  progress  toward  the  east, 
it  is  evident  that  some  winds  must  be  propagated  from  place  to 
place  nearly  in  the  same  direction  as  that  in  which  they  blow, 
while  others  are  propagated  in  a  direction  opposite  to  that  in  which 
they  blow.     When  a  great  storm  springs  up  near  the  Mississippi, 
the  wind  at  St.  Louis  is  generally  easterly,  while  throughout  New 
York  and  Ohio  the  wind  is  from  the  west.     Subsequently  this 
easterly  wind  is  felt  at  Cincinnati,  then  at  Pittsburg,  and  after- 
ward at  New  York,  while  the  entire  storm  is  traveling  steadily 
eastward ;  that  is,  the  easterly  wind  is  propagated  from  St.  Louis 
to  New  York  in  a  direction  opposite  to  that  in  which  the  wind 
blows. 

After  the  centre  of  the  storm  has  passed,  a  west  wind  springs 
up  at  St. Louis,  and  this  west  wind  is  felt  successively  at  Cincin- 
nati, Pittsburg,  and  finally  at  New  York,  having  been  propa- 


STORMS,    TORNADOES,   AND   WATER-SPOUTS.  145 

gated  in  the  same  direction  as  that  in  which  the  wind  blows. 
The  former  wind  is  said  to  be  propagated  by  aspiration,  the  latter 
by  impulsion,  as  stated  in  Art.  142. 

277.  Temperature  near  the  Centre  of  a  Storm. — During  an  exten- 
sive rain-storm  the  temperature  of  the  air  generally  rises  above 
its  mean  height  for  that  season  of  the  year.     This  increase  of  tem- 
perature frequently  amounts  to  10°  or  20°,  and  sometimes  even 
30°.     This  is  caused  by  the  latent  heat  which  is  liberated  from  the 
vapor  when  it  is  condensed  into  water.     The  centre  of  the  area 
of  high  thermometer  frequently  does  not  coincide  with  that  of  the 
area  of  low  barometer,  or  with  the  centre  of  the  area  of  rain  and 
snow.     In  the  United  States,  on  the  northeast  side  of  a  storm,  at 
a  distance  of  over  500  miles  from  the  area  of  rain  and  snow,  the 
thermometer  sometimes  rises  even  20°  above  its  mean  height.     It 
seems  probable  that  the  heat  which  is  liberated  in  the  condensa- 
tion of  the  vapor  expands  the  upper  portion  of  the  atmosphere, 
and  is  drifted  eastward  far  in  advance  of  the  storm. 

278.  Low  Temperature  succeeding  a  Storm. — On  the  western  side 
of  the  storrn  centre,  a  cold  wind  presses  in  from  the  north  or  north- 
west, by  which  the  temperature  is  reduced  10  or  20  degrees  below 
the  mean.     Thus,  when  a  storm  is  prevailing  in  the  middle  of  the 
United  States,  the  lowest  temperature  of  the  month  may  occur  at 
St.  Louis  on  the  same  day  that  the  highest  temperature  occurs  at 
New-York. 

279.  Motion  of  the  Air  within  Areas  of  Low  and  High  Barometer. 
— It  is  found  that  within  the  limits  of  the  United  States  around 
every  storm-centre  the  wind  moves  spirally  inward,  circulating 
about  the  centre  in  a  direction  contrary  to  the  motion  of  the 
hands  of  a  watch,  and  the  average  inclination  of  the  winds  to 
a  radius  drawn  from  the  centre  of  the  storm  is  almost  exactly 
45°. 

On  the  contrary,  within  an  area  of  high  barometer  the  winds 
blow  outward,  and  the  average  inclination  of  the  winds  to  a  radius 
drawn  from  the  centre  of  the  area  is  also  about  45°,  but  the  winds 
circulate  about  the  centre  of  the  area  in  the  same  direction  as  that 
of  the  hands  of  a  watch.  The  former  motion,  being  in  the  same 
direction  as  that  observed  in  violent  cyclones,  is  called  cyclonic; 

K 


146  METEOROLOGY. 

and  the  latter  motion,  being  in  the  opposite  direction,  is  called 
anti-cyclonic. 

Over  the  ocean,  where  the  air  encounters  less  resistance,  the 
motion  of  the  air  around  a  storm-centre  is  more  nearly  circular, 
which  accords  with  the  views  promulgated  fifty  years  ago  by  "W. 
C.  Redfield.  The  first  tendency  of  the  air,  however,  appears  to 
be  always  inward  toward  the  area  of  rain,  and  this  principle 
was  first  distinctly  announced  by  J.  P.  Espy  nearly  forty  years 
ago. 

280.  Storms  traced  across  the  Atlantic. — Between  the  parallels 
of  40°  and  60°  of  north  latitude,  especially  during  the  colder 
months,  there  is  an  almost  uninterrupted  succession  of  barometric 
waves  traveling  from  the  Rocky  Mountains  to  the  Atlantic,  thence 
across  the  Atlantic  to  Europe,  across  Europe  into  Asia,  and  prob- 
ably also  across  Asia  and  the  Pacific  Ocean,  thus  completing  the 
circuit  of  the  globe.     But  these  waves  undergo  important  modi- 
fications from  day  to  day  in  consequence  of  changes  going  on 
within  each  storm-area,  and  in  consequence  of  the  interference 
of  different  storms  with  each  other,  so  that  we  can  seldom  identify 
an  individual  storm  in  its  progress  for  more  than  a  week.     In  a 
few  cases,  American  storms  have  been  satisfactorily  traced  across 
the  Atlantic  to  Europe,  but  in  general  the  storms  of  Europe  are 
quite  distinct  from  those  of  America. 

281.  Influence  of  Local  Causes. — Local  causes  sometimes  exert 
an  important  influence  upon  storms.     A  high  mountain  near  the 
path  of  a  storm,  by  deflecting  the  air  upward,  may  cause  an  in- 
creased condensation  of  vapor,  give  increased  force  to  the  winds, 
and  produce  a  corresponding  depression  of  the  barometer.     In 
such  a  case  the  storm  is  often  detained  in  its  course  for  one  or 
more  days,  its  centre  remaining  nearly  stationary  over  the  mount- 
ain.    This  effect  will  be  the  more  remarkable  if  near  the  mount- 
ain there  is  a  large  body  of  warm  water  to  supply  vapor  to  the 
atmosphere.    Thus  the  vapor  from  the  Gulf  Stream  is  precipitated 
on  the  mountains  of  Iceland,  and  the  storms  in  this  vicinity  are 
remarkable  for  their  violence  and  long  continuance.     A  similar 
effect  is  produced  in  the  vicinity  of  Newfoundland,  where  storms 
of  great  violence  have  been  known  to  remain  almost  stationary 
for  four  or  five  days. 


STORMS,    TORNADOES,    AND   WATER-SPOUTS.  147 

282.  Cause  of  the  Low  Barometer  near  the  Equator. — The  same 
principles  which  are  developed  in  the  action  of  storms  are  ex- 
emplified on  a  grand  scale  in  the  general  circulation  of  the  atmos- 
phere.    The  N.E.  and  S.E.  trade  winds,  encountering  each  other 
near  the  equator,  are  forced  up  to  a  great  height,  where  their  vapor 
is  condensed ;  copious  rain  follows;  by  the  liberated  heat  the  air  is 
expanded,  and  flows  off  laterally  from  above.     This  causes  the 
barometer  to  fall  at  the  equator,  and  to  rise  at  some  distance  on 
each  side  of  the  equator. 

283.  Low  Barometer  near  Iceland. — During  winter  the  storms 
of  the  North  Atlantic  Ocean  are  extremely  severe,  and  their  cen- 
tres generally  pass  near  Iceland.     These  storms  are  attended  by 
a  very  low  pressure,  the  barometer  frequently  falling  below  29 
inches,  and  sometimes  below  28.5  inches.     This  low  pressure  is 
well-nigh  uninterrupted,  and  seldom  alternates  with  high  pressure; 
so  that  the  mean  pressure  of  January  in  the  neighborhood  of 
Iceland  is  reduced  to  29.4  inches.     In  summer  the  storms  are 
less  severe,  and  the  depression  of  the  barometer  is  not  so  great. 

284.  Cause  of  the  Uniformity  of  the  Monsoons. — The  uniformity 
and  strength  of  the  S.W.  monsoon  in  India,  described  in  Art.  152,  is 
due  to  the  vast  amount  of  vapor  precipitated  on  the  Himalaya 
Mountains.     The  heat  which  is  liberated  in  this  condensation 
causes  the  air  over  the  mountains  to  expand  and  flow  off  in  the 
higher  regions  of  the  atmosphere,  causing  a  greatly  diminished 
pressure  in  the  lower  atmosphere,  and  this  cause  converts  the  S.W. 
wind  of  India,  which  otherwise  might  be  a  feeble  and  variable 
wind,  into  a  strong  and  permanent  wind  throughout  the  warmer 
months  of  the  year. 

SECTION   II. 

CYCLONES. 

285.  Cyclones  defined. — The  inequalities  of  the  earth's  surface, 
especially  in  hilly  countries,  greatly  modify  the  direction  of  the 
wind,  so  that  in  great  storms  the  movements  of  the  atmosphere 
often  seem  very  complex  and  anomalous.     Over  the  ocean  these 
disturbing  causes  do  not  exist,  and  here  we  find  that  in  violent 
storms  the  movements  of  the  air  are  much  more  regular  and  unr 


148  METEOROLOGY. 

form.  This  motion  of  the  wind  has  generally  been  found  to  be 
in  great  circuits,  spirally  inward  toward  the  centre  of  the  storm, 
and  such  storms  are  now  commonly  designated  by  the  term 
cyclone.  These  storms  prevail  in  the  neighborhood  of  the  West 
India  Islands,  where  they  have  long  been  known  by  the  name  of 
hurricanes.  They  are  also  common  in  the  China  Sea  and  in  the 
Indian  Ocean,  on  both  sides  of  the  equator. 

286.  /Season  of  Cyclones. — In  the  West  Indies,  cyclones  are  al- 
most exclusively  confined  to  the  months  from  July  to  October, 
being  most  common  in  the  month  of  August.     In  the  China  Sea 
and  the  Bay  of  Bengal,  they  occur  in  all  months  of  the  year  from 
April  to  November,  but  they  are  most  common  in  the  autumn.  In 
southern  latitudes  they  are  most  common  from  January  to  March. 

287.  Where  do  Cyclones  originate? — There  is  no  instance  on 
record  of  a  cyclone  having  been  encountered  on  the  equator, 
nor  of  any  one  having  crossed  that  line,  although  two  have  been 
known  to  rage  at  the  same  time  on  the  same  meridian,  but  on  op- 
posite sides  of  the  equator,  and  10°  or  12°  apart.    They  originate 
near  the  equatorial  limit  of  the  trade  winds,  where  these  winds  are 
irregular.    The  West  India  cyclones  generally  originate  between 
lat.  10°  and  20° N.,  and  long.  50°  and  60°  W.,  on  the  borders  of  the 
zone  of  calms  and  variable  winds,  which,  corresponds  with  the 
zone  of  constant  precipitation  of  rain. 

288.  Paths  of  Cyclones. —  In  the  northern  hemisphere,  during 
the  early  part  of  their  course  within  the  region  of  the  trade  winds, 
cyclones  travel  toward  the  west,  inclining  somewhat  toward  the 
north.     Near  lat.  20°  the  motion  from  the  equator  is  more  de- 
cided, and  in  lat.  25°  their  motion  is  about  N.W.     Near  the  par- 
allel of  30°  their  course  is  almost  exactly  north,  and  soon  they  be- 
gin to  veer  toward  the  east,  after  which  their  motion  is  nearly 
parallel  to  the  coast  of  the  United  States.     Several  storms  have 
been  traced  from  lat.  10°  or  15°  up  to  lat.  45°  or  50°,  and  the 
path  of  the  centre  of  greatest  violence  bears  some  resemblance  to 
a  parabola,  of  which  the  most  westerly  point  lies  near  the  parallel 
of  30°.     This  path  is  represented  by  the  line  ABC,  Fig.  61. 

In  the  southern  hemisphere  cyclones  pursue  a  similar  course. 
Commencing  near  latitude  10°,  they  advance  at  first  only  a  little 


STORMS,    TORNADOES,    AND   WATER-SPOUTS. 


U9 


south  of  west.  This 
southerly  motion  in- 
creases until  near  lat. 
26°,  when  the  motion 
is  exactly  toward  the 
south,  after  which  they 
gradually  veer  toward 
the  southeast,  the  en- 
E  tire  path,  DEG,  form- 
ing a  curve  which  is 
almost  perfectly  sym- 
metrical with  that  of 
cyclones  in  the  north- 
ern hemisphere.  The 
latitude  where  the 
path  of  the  cyclone 

changes  from  west  to  east  coincides  nearly  with  the  polar  limit 

of  the  trade  winds. 

289.  Gyratory  Movement  of  Cyclones. — The  air  in  cyclones  has 
not  merely  a  movement  of  translation,  but  also  a  gyratory  motion 
about  the  centre  of  the  storm.     The  motion  of  the  air  is  spirally 
inward,  as  has  been  already  shown  in  the  storms  of  the  United 
States,  but  over  the  ocean  the  whirling  motion  is  usually  more 
decided  than  it  is  over  the  land.     North  of  the  equator  this  gyra- 
tory motion  is  from  right  to  left,  or  in  a  direction  contrary  to  that 
of  the  hands  of  a  watch.     South  of  the  equator  the  motion  is  from 
left  to  right,  or  in  the  same  direction  as  that  of  the  hands  of  a 
watch. 

Near  the  centre  of  the  hurricane  there  is  generally  a  great  fall 
of  rain,  which  is  usually  accompanied  by  the  most  magnificent 
displays  of  thunder  and  lightning. 

290.  Rate  of  Motion. — The  rate  at  which  cyclones  travel  is  very 
variable.     In  the  West  India  cyclones  the  highest  rate  which  has 
been  observed  is  35  miles  per  hour,  and  the  least  10  miles  per 
hour;  the  mean  being  18  miles.     In  the  Bay  of  Bengal  the  ob- 
served rate  varies  from  2  to  39  miles  per  hour,  and  in  the  China 
Sea  from  7  to  24  miles  per  hour.     In  the  South  Indian  Ocean  the 
observed  rate  varies  from  1  to  10  miles  per  hour.     Some  cyclones 


150  METEOROLOGY. 

travel  so  very  slowly  that  they  may  almost  be  considered  sta- 
tionary. 

The  direction  and  velocity  of  the  wind  are,  however,  entirely 
distinct  from  those  of  the  storm's  progress.  While  the  storm 
sometimes  advances  at  the  rate  of  less  than  10  miles  per  hour, 
the  velocity  of  the  wind  may  exceed  100  miles  per  hour. 

291.  Diameter  of  Cyclones. — Cyclones  extend  over  a  circle  from 
100  to  500  miles  in  diameter,  and  sometimes  1000  miles.     In  the 
West  Indies  they  are  sometimes  as  small  as  100  miles  in  diame- 
ter, but  on  reaching  the  Atlantic  they  dilate  to  600  or  1000  miles. 
Sometimes,  on  the  contrary,  they  contract  in  their  progress,  and 
while  contracting  they  augment  fearfully  in  violence.     The  vio- 
lence of  the  wind  increases  from  the  margin  to  the  centre,  with 
the  exception  of  a  limited  space  exactly  at  the  centre,  where  the 
atmosphere  is  frequently  quite  calm. 

292.  Premonitions  of  a  Cyclone. —  Previous  to  the  commence- 
ment of  a  cyclone  the  air  is  observed  to  be  close,  sultry,  and  op- 
pressive, and  the  wind  is  moderate  or  calm.     A  fresh  breeze  sets 
in  from  the  east,  and  rises  and  falls  with  a  moaning  sound  ;  after 
a  few  hours  it  is  succeeded  by  a  lull,  which  may  last  for  an  hour 
or  more,  after  which  the  wind  changes  to  the  west,  often  with 
great  suddenness,  and  blows  with  increased  violence,  and  this  is 
usually  the  time  of  greatest  danger  to  vessels. 

The  approach  of  a  cyclone  is  often  announced  by  a  swell  of  the 
ocean,  resulting  from  the  action  of  the  wind  upon  a  neighboring  sea, 
while  the  waves  thus  excited  advance  more  rapidly  than  the  storm. 

During  the  passage  of  the  cyclone  the  barometer  oscillates  in  a 
remarkable  manner,  rising  and  falling  rapidly,  so  that  a  great 
barometric  oscillation  almost  always  announces  the  approach  of 
a  tempest.  The  most  rapid  fall  begins  from  three  to  six  hours 
before  the  passage  of  the  centre.  The  barometer  is  lowest  near 
the  middle  of  the  storm  area,  and  begins  to  rise  before  the  strength 
of  the  cyclone  is  over. 

The  fall  of  the  barometer  during  the  passage  of  the  cyclone 
varies  according  to  the  intensity  of  the  storm.  It  frequently 
amounts  to  one  inch,  and  has  been  known  to  exceed  two  inches. 
The  rise  of  the  barometer  after  the  storm  is  usually  as  rapid  as 
was  its  fall  on  the  approach  of  the  storm.  See  Table  XXXIII. 


STORMS,   TORNADOES,    AND   WATER-SPOUTS.  151 

293.  Duration  at  any  Place. — The  duration  of  the  storm  at  any 
p.ace  depends  upon  the  extent  of  the  storm,  and  the  velocity  with 
which  it  advances.    If  the  storm  be  only  100  miles  in  diameter,  and 
advances  20  miles  per  hour,  its  duration  at  any  place  can  not  ex- 
ceed five  hours.     If  the  diameter  of  the  storm  be  greater,  or  its 
progress  less   rapid,  its   duration   at  a  given  place  will  be   in- 
creased. 

294.  Cattle  of  the  Parabolic  Course  of  Storms, — The  parabolic 
course  of  storms  from  near  the  equator  toward  the  poles  results 
from  the  rotary  motion  of  the  earth.     When  a  large  mass  of  air 
in  the  northern  hemisphere  is  put  in  rotation  about  a  vertical  axis, 
the  particles  on  the  east  side  of  the  centre,  crossing  successively 
parallels  of  latitude  whose  easterly  motion  is  less  than  their  own, 
are  deflected  toward  the  east ;  that  is,  toward  the  right.     So,  also, 
the  particles  on  the  west  side  of  the  centre,  crossing  successively 
parallels  of  latitude  whose  easterly  motion  is  greater  than  their 
own,  are  deflected  toward  the  west,  which  is  also  toward  the  right. 
Particles  on  the  north  or  south  side  of  the  centre  are  deflected  in 
a  similar  manner;  that  is,  the  particles  of  the  revolving  mass  of 
air,  in  every  portion  of  their  circuit,  are  deflected  toward  the  right. 
Hence  on  the  equatorial  side  of  the  revolving  mass  of  air  there 
is  a  tendency  toward  the  equator,  while  on  the  polar  side  there 
is  a  tendency  toward  the  pole.      Now  this  deflecting  force  in- 
creases from  the  equator  toward  the  pole,  being  proportional  to 
the  sine  of  the  latitude.     Hence  the  pressure  on  the  polar  side 
toward  the  pole  is  greater  than  on  the  opposite  side  toward  the 
equator,  and  the  revolving  mass  accordingly  moves  in  the  direc- 
tion of  greatest  pressure ;  that  is,  toward  the  pole. 

Within  the  limit  of  the  trade  winds  the  revolving  mass  is  car- 
ried westward  by  the  general  westward  motion  of  the  atmosphere, 
while  it  is  crowded  northward  by  the  force  just  described,  so  that 
the  actual  progress  of  the  storm  is  toward  the  north  of  west.  Aft- 
er escaping  from  the  trade  winds,  the  general  motion  of  the  at- 
mosphere carries  the  storm  eastward,  while  the  force  just  described 
urges  it  northward;  that  is,  the  actual  progress  of  the  storm  is  to- 
ward the  north  of  east. 

By  a  similar  course  of  reasoning,  the  parabolic  path  of  cyclonea 
in  the  southern  hemisphere  may  be  explained. 


152  METEOROLOGY. 

SECTION  III. 

TORNADOES. 

295.  Sometimes  near  the  centre  of  a  great  storm  the  general 
inward  tendency  of  the  air  causes  a  violent  whirlwind,  or  tornado, 
where  the  wind  revolves  with  such  violence  as  to  prostrate  the 
largest  trees,  demolish  buildings,  and  transport  heavy  bodies  to  a 
great  distance.     Such  a  whirlwind  occurred  in  Northern  Ohio 
February  4, 1842,  near  the  centre  of  an  uncommonly  severe  storm 
of  rain.     In  this  tornado  large  buildings  were  lifted  entire  from 
their  foundations,  carried  a  distance  of  several  rods,  and  then 
dashed  to  pieces.     The  fragments  were  strewed  all  along  the 
track,  and  some  were  carried  a  distance  of  seven  or  eight  miles. 
Large  oak-trees,  two  feet  in  diameter,  were  snapped  off  like  reeds, 
and  others  were  so  twisted  as  to  be  reduced  to  a  mass  of  splinters 
not  much  thicker  than  a  man's  finger.     The  breadth  of  the  track 
did  not  much  exceed  half  a  mile,  and  the  most  destructive  portion 
was  still  more  limited.     The  duration  of  the  tornado  at  one  place 
did  not  much  exceed  one  minute.     The  tornado  advanced  over 
the  earth,  in  a  direction  N.33°  E.,  with  a  velocity  of  34  miles  per 
hour. 

296.  Tropical  Tornadoes. — Similar  tornadoes  occur  within  the 
tropics,  and  here  exhibit  even  greater  violence  than  they  do  in 
the  United  States.     In  the  great  tornado  which  passed  overBar- 
badoes  in  1780,  the  strongest  buildings  were  entirely  demolished; 
the  largest  trees  were  torn  up  by  the  roots ;  a  12-pounder  gun 
was  moved  a  distance  of  140  yards;  a  multitude  of  ships  were 
wrecked,  and  over  4000  persons  perished. 

In  a  hurricane  which  occurred  in  June,  1822,  near  the  mouth 
of  the  Ganges,  a  vast  amount  of  property  was  destroyed,  and  up- 
ward of  50,000  persons  perished,  chiefly  from  the  inundation  of 
the  rivers. 

297.  Effects  of  Tornadoes. — The  motion  of  the  air  in  tornadoes 
is  spirally  inward  and  upward,  so  that  from  each  side  of  the 
track  objects  are  drawn  inward  toward  the  centre  of  the  track, 
and  very  heavy  bodies  are  carried  up  in  the  centre.     Light  ob- 
jects are  elevated  high  into  the  air,  and  are  sometimes  carried 


STORMS,   TORNADOES,   AND  WATER-SPOUTS. 


153 


Fig.  62.  many  miles  before  they  are 

,x'    thrown  out  of  the  vortex. 

Fig.  62  represents  a  por- 
tion of  the  track  of  a  tornado 
which  passed  over  New  Ha- 
ven in  1839.  The  tornado 
advanced  in  a  direction  N. 
50°  E.  On  the  right-hand 
side  of  the  track  the  pros- 
trate trees  were  uniformly 
inclined  toward  the  north, 
while  on  the  left-hand  side 
many  of  them  were  inclined 
toward  the  south. 

Tornadoes  are  uniformly 
preceded  by  an  unusual  heat; 
they  are  invariably  accompanied  by  lightning  and  rain,  and  fre- 
quently by  hail. 

When  a  tornado  passes  over  a  hilly  country,  it  sometimes  rages 
with  destructive  violence  on  the  hill-tops,  while  objects  in  the  in- 
termediate valleys  are  entirely  uninjured,  showing  that  a  violent 
whirlwind  may  prevail  at  a  moderate  elevation,  but  without  reach- 
ing the  earth's  surface. 

298.  Appearance  of  Explosion. — When  a  violent  tornado  passes 
over  a  building  where  the  doors  and  windows  are  closed,  the 
walls  are  sometimes  thrown  outward  with  great  force,  the  house 
presenting  the  appearance  of  an  explosion,  indicating  that  the 
pressure  of  the  air  on  the  outside  of  the  building  was  suddenly 
diminished,  and  the  house  was  burst  open  by  the  expansion  of 
the  air  within. 


SECTION  IV. 

PILLARS   OF   SAND,  AND   WATER-SPOUTS. 

299.  Tornadoes  are  probably  similar  to  the  small  whirls  which 
are  often  seen  in  the  streets,  especially  on  dry  and  calm  days  of 
spring  or  summer,  and  which  raise  up  a  dense  column  of  dust, 
even  to  the  tops  of  the  houses.  In  these  whirls  the  motion  of  the 
air  is  spirally  inward  and  upward,  so  that  light  objects  in  their 


154  METEOROLOGY. 

vicinity  are  sucked  into  the  vortex,  and  carried  up  to  the  top  of 
the  whirl,  where  they  escape  laterally,  and  descend  at  some  dis- 
tance on  either  side.  These  small  whirls  sometimes  revolve  from 
left  to  right,  and  sometimes  from  right  to  left,  while  in  the  north- 
ern hemisphere  large  whirlwinds,  several  miles  in  diameter,  al- 
ways revolve  from  right  to  left.  The  whirls  seen  in  our  streets 
are  sometimes  only  a  few  inches  in  diameter,  but  sometimes  in  the 
open  fields  they  occur  several  feet  in  diameter,  and  carry  up  leaves 
of  trees  and  light  objects  of  considerable  size. 

On  the  deserts  of  Africa  similar  whirls  often  raise  vast  pillars 
of  sand,  which  sometimes  prove  fatal  to  entire  caravans.  Bruce 
states  that  in  Abyssinia  he  beheld  eleven  vast  columns  of  sand 
moving  over  the  plain  at  the  same  time.  Similar  whirls  are  of 
common  occurrence  in  India. 

300.  Whirlwinds  caused  by  Fires. — These  whirls  may  be  set  in 
motion  by  whatever  causes  a  strong  upward  motion  of  the  air. 
An  extensive  fire  frequently  produces  this  effect.     When  large 
fires  are  burning  on  the  Western  prairies,  violent  whirls  are  fre- 
quently formed,  having  a  force  sufficient  to  lift  a  man  from  the 
ground  and  transport  him  to  a  considerable  distance.     At  such 
times  the  flame  is  sometimes  collected  into  a  fiery  column,  rising 
to  the  height  of  200  feet  or  more. 

Some  years  since,  during  the  burning  of  a  canebrake  in  Ala- 
bama, several  whirls  were  formed  in  the  midst  of  the  flames,  some 
of  which  rose  to  the  height  of  200  feet,  and  in  form  resembled  the 
upper  cone  of  an  hour-glass. 

Similar  effects  were  produced  by  the  conflagration  of  Moscow, 
September  14-20, 1812. 

301.  Water-spouts. — When  a  violent  whirl  is  formed  over  water, 
considerable  spray  is  raised  from  the  surface  of  the  water,  and 
this  spray  is  carried  up  in  the  centre  of  the  whirl,  presenting  the 
appearance  of  a  dense  solid  column.     This  phenomenon  is  called 
a  water-spout.     Water-spouts   are   of  variable   dimensions,  but 
sometimes  they  attain  a  diameter  of  several  rods,  and  a  height  of 
half  a  mile. 

These  whirls  generally  form,  in  the  first  instance,  at  a  consider- 
able height  in  the  air,  and  do  not  reach  down  to  the  surface  of 
the  sea.  If  there  is  a  low  cloud  over  it,  the  under  surface  of  the 


STORMS,   TORNADOES,    AND   WATER-SPOUTS. 


155 


cloud  is  rolled  into  a  conical  form.  This  inverted  cone  seems  at- 
tached to  the  cloud,  and  sometimes  becomes  rapidly  elongated. 
Sometimes  it  swings  backward  and  forward,  coils  up,  and  disap- 
pears, and  the  spout  is  not  completed;  but  at  other  times  it  grad- 
ually extends  so  as  to  reach  down  to  the  surface  of  the  water. 
As  the  column  approaches  the  surface  of  the  sea,  the  latter  be- 
comes violently  agitated,  and  the  spray  is  whirled  round  with  a 
rapid  motion.  The  spout  now  forms  a  continuous  column,  ex- 
tending from  the  "water  to  the  cloud,  and  often  resembles  a  large 
elephant's  trunk  dangling  from  the  clouds.  Its  color  is  generally 
of  a  sombre  gray,  like  that  of  the  clouds,  but  sometimes  it  appears 
black,  like  a  dense  smoke. 

This  spout  has  both  a  rotary  and  a  progressive  motion.  The 
whirling  motion  extends  to  but  a  moderate  distance  around  the 
column,  and  beyond  this  there  prevails  a  calm.  The  phenomenon 
lasts  but  a  short  time.  After  a  few  minutes  the  trunk  contracts 
so  as  no  longer  to  reach  the  surface  of  the  sea,  the  black  cloud 
draws  itself  up,  and  the  trunk  gradually  disappears.  Sometimes 
the  spout  commences  with  the  rising  of  spray  from  the  surface  of 
the  water,  which  gradually  ascends  until  the  column  is  complete 
from  the  water  to  the  clouds.  When  the  spout  is  complete,  there 
is  heard  a  roaring  noise  like  that  of  a  great  waterfall. 


Fier.  63. 


156 


METEOROLOGY. 


Subsequently  the  cloud  sometimes  discharges  itself  in  a  heavy 
rain,  and  this  rain  is  never  salt,  even  in  the  open  ocean,  showing 
that  this  water  was  precipitated  from  the  clouds,  as  in  ordinary 
rains.  Fig.  63,  on  the  preceding  page,  shows  a  water -spout  in 
three  stages  of  its  progress.  First,  the  column  is  incomplete; 
next,  the  column  is  entire;  and,  finally,  the  smoky  aspect  of  the 
column  disappears,  and  the  column  begins  to  break  up. 

Water-spouts  generally  form  during  a  period  of  great  heat,  and 
are  most  frequent  in  the  cairn  regions  between  the  tropics.  Two 
or  three  of  these  spouts  are  sometimes  formed  simultaneously, 
proceeding  from  the  same  cloud.  In  May,  1820,  on  the  edge  of 
the  Gulf  Stream,  seven  water-spouts  were  seen  in  the  course  of 

Fig.  64. 


half  an  hour.     Fig.  64  represents  a  water-spout  seen  in  1858  on 
the  River  Rhine. 

302.  Showers  of  Toads,  Fishes,  etc. — During  violent  storms  show- 
ers of  small  animals  sometimes  descend  from  the  sky.  M.  Peltier, 
of  France,  states  that  he  once  saw  a  multitude  of  small  toads  de- 
scend to  the  earth.  They  fell  upon  his  hat,  upon  his  hands,  and 
the  ground  about  him  was  covered  with  them.  Several  observ- 
ers in  France,  in  India,  and  elsewhere,  have  seen  showers  of  small 


STORMS,    TORNADOES,    AND  WATER-SPOUTS.  157 

fish  descend  from  the  sky.  Others  have  observed  showers  of 
sand,  of  straws,  etc. 

These  phenomena  are  explained  by  supposing  that  the  objects 
mentioned  were  elevated  from  the  earth  in  a  violent  whirl,  which 
transported  them  to  a  considerable  distance,  and  then  dropped 
them  upon  the  earth. 

In  1833,  near  Naples,  a  whirlwind  passed  over  an  orange-grove, 
and  a  multitude  of  oranges  were  carried  up  in  the  whirl.  Some 
minutes  afterward  a  shower  of  oranges  fell  upon  a  roof  at  a  con- 
siderable distance. 

In  1835,  in  France,  the  water  of  a  small  pond  containing  a  large 
quantity  of  fish  was  drawn  off  by  a  whirlwind.  These  animals 
may  have  been  transported  a  distance  of  many  rods,  perhaps 
several  miles,  but  they  must  ultimately  have  fallen  to  the  earth, 
furnishing  a  shower  of  fishes. 

SECTION  V. 

PREDICTIONS   OF  THE   WEATHER. 

303.  The  character  of  the  weather  at  any  place  is  affected  by 
so  many  circumstances  which  may  transpire  at  distant  parts  of 
the  world,  and  which  can  be  but  very  imperfectly  known  to  us, 
that  it  is  impossible  to  predict,  except  very  imperfectly,  what 
weather  may  be  expected  at  a  given  time  and  place.     To  a  limit- 
ed extent,  however,  such  predictions  are  possible. 

304.  Predictions  founded  upon  the  Constancy  of  Climate. — Rely- 
ing upon  the  constancy  of  climate,  which  has  been  established  by 
observation,  we  may  predict  the  probable  general  character  of 
any  month  of  the  year. 

The  climate  of  a  country  remains  permanently  the  same  from 
age  to  age.  Observations  continued  for  an  entire  century  at  va- 
rious places  in  the  United  States  and  Europe  indicate  no  change 
in  the  mean  temperature  of  the  year,  or  that  of  the  separate 
months  ;  no  change  in  the  range  of  the  thermometer;  no  change 
in  the  time  of  the  last  frost  of  spring  or  the  first  frost  of  autumn ; 
in  the  annual  amount  of  rain  or  snow,  or  in  the  mean  direction  of 
the  wind.  It  is  not  certain  that  the  climate  of  any  country,  in 
either  of  these  respects,  has  changed  appreciably  in  2000  years. 
By  the  destruction  of  forests,  the  earth  is  more  directly  exposed 


158  METEOROLOGY. 

to  the  rays  of  the  sun  ;  the  moisture  of  the  ground  is  more  readi- 
ly evaporated ;  streams  more  frequently  dry  up  in  summer,  and 
droughts  become  more  frequent  and  severe.  But  these  changes 
do  not  seem  to  affect  in  a  sensible  manner  the  mean  temperature 
of  any  place,  or  the  annual  amount  of  rain. 

Assuming,  then,  the  established  constancy  of  climate,  we  can 
predict  beforehand  the  probable  character  of  any  month  o'f  the 
year.  Thus,  at  New  Haven,  the  probable  mean  temperature  of 
any  future  January  will  be  26°.  We  may  be  tolerably  sure  that 
it  will  not  be  higher  than  36°,  nor  lower  than  17°.  The  ther- 
mometer in  January  will  never  rise  above  64°,  nor  sink  below 
—  24°.  The  entire  annual  amount  of  rain  at  New  Haven  will  not 
exceed  55  inches,  and  will  not  be  less  than  34  inches. 

305.  Conclusions  drawn  from  anomalous  Months. — Moreover,  if 
several  months  in  succession  have  been  unusually  warm  or  unu- 
sually cold,  instead  of  concluding  that  the  climate  has  permanently 
changed,  and  that  the  succeeding  months  will  be  similar  in  char- 
acter, we  should  rather  anticipate  months  of  the  opposite  descrip- 
tion, since  the  mean  temperature  of  the  year  fluctuates  within 
very  narrow  limits,  and  the  longer  a  period  of  unusually  warm 
weather  continues,  the  greater  is  the  probability  that  the  succeed- 
ing months  will  be  unusually  cold.     Predictions  of  this  kind  are 
legitimate  deductions  from  scientific  data. 

306.  Predictions  founded  upon  the  established  Laws  of  Storms. — 
The  laws  of  storms  are  now  so  well  understood  that  we  can  pre- 
dict with  some  confidence  the  changes  which  will  succeed  at  any 
place  during  the  next  few  hours,  provided  we  can  know  the  state 
of  the  weather  throughout  the  surrounding  region  to  a  great  dis- 
tance.   This  is  what  has  been  attempted  since  1871  by  the  United 
States  Signal  Service,  and  the  general  accuracy  of  these  predic- 
tions has  excited  considerable  surprise.     Such  predictions  would 
be  still  more  reliable  if  we  could  have  information  respecting  the 
various   meteorological  elements  from  a  larger  portion  of  the 
earth's  surface.     The  centre  of  a  large  portion  of  our  storms  fol- 
lows nearly  the  northern  boundary  of  the  United  States,  so  that 
our  observations  inform  us  respecting  only  one  half,  or  perhaps 
less  than  one  half  of  the  storm-area.     Moreover,  storms  are  often 
affected  by  changes  which  are  going  on  in  very  distant  quarters. 


STORMS,  TORNADOES,  AND  WATER-SPOUTS.  159 

An  ar'ea  of  unusually  high  barometer  may  affect  the  course  of  a 
storm  whose  centre  is  distant  two  or  three  thousand  miles;  and 
an  unusual  fall  of  rain  in  the  equatorial  regions  may  cause  an  un- 
usual overflow  of  air  to  the  middle  latitudes,  resulting  in  serious 
disturbances  of  atmospheric  pressure.  When  the  laws  of  storms 
have  been  more  precisely  defined,  and  telegraphic  reports  can  be 
received  from  a  more  extended  area,  we  shall  doubtless  be  able 
to  predict  coming  storms  with  greater  precision. 

307.  Predictions  from  Observations  at  a  Single  Place. — When  an 
observer  is  required  to  predict  changes  of  the  weather  from  ob- 
servations made  at  a  single  locality,  the  results  are  much  more 
uncertain ;  nevertheless,  even  such  observations  will  often  enable 
us  to  anticipate  the  approach  of  a  storm.    Near  the  Atlantic  coast 
of  the  United  States  the  approach  of  a  storm  is  usually  indicated 
by  the  barometer  rising  above  its  mean  height;  the  wind  veers 
to  the  N.E. ;  the  atmosphere  grows  hazy,  and  the  haze  gradually 
thickens  until  the  entire  sky  is  overcast  with  a  dense  stratus.    If 
the  wind  increases  in  force  from  some  eastern  quarter,  and  the 
barometer  begins  to  fall,  rain  usually  follows  in  a  few  hours,  or 
snow  if  the  thermometer  does  not  stand  above  32°.     If  the  wind 
sets  in  from  the  S.E.,  the  temperature  will  be  above  the  mean  for 
that  season  of  the  year,  and  except  in  the  extreme  northern  part 
of  the  United  States,  the  result  will  be  rain. 

308.  Prognostics  from  the  Clouds,  Face  of  the  Sky,  etc. — When 
the  upper  clouds  move  in  a  direction  different  from  that  of  the 
lower  clouds,  or  that  of  the  wind  prevailing  at  the  surface  of  the 
earth,  they  foretell  a  change  of  wind.     Remarkable  clearness  of 
the  atmosphere,  and  an  unusual  twinkling  of  the  stars,  indicate 
unusual  humiditj^  in  the  upper  regions  of  the  atmosphere,  and 
afford  reason  to  anticipate  rain.    A  light  scud  driving  across  hazy 
clouds  foretells  wind  and  rain.     Halos,  coronas,  etc.,  indicate  a 
precipitation  of  vapor  in  the  upper  regions  of  the  atmosphere, 
and  foretell  approaching  rain  or  snow. 

When  the  outlines  of  cumulus  clouds  are  sharp,  it  indicates  a 
dry  atmosphere,  and  therefore  presages  fine  weather. 

Fogs  do  not  generally  result  in  rain.  A  fog  which  forms  dur- 
ing the  night  is  generally  dissipated  by  the  rising  sun,  and  a 
foggy  morning  is  followed  by  a  pleasant  day. 


L60 


METEOROLOGY. 


CHAPTER  VII 

ELECTRICAL  PHENOMENA. 

SECTION  I. 

ATMOSPHERIC    ELECTRICITY. 

309.  Means  of  observing  the  Electricity  of  the  Atmosphere. — 
atmosphere  is  almost  always  charged  with  electricity,  and  this 
electricity  exerts  an  important  influence  upon  various  meteoro- 
logical phenomena. 

In  order  to  observe  this  electricity,  an  insulated  conductor 
should  be  elevated  to  a  considerable  height  above  the  earth.  At 
the  Observatory  of  Kew,  near  London,  a  tube  of  thin  copper,  16 
feet  high,  and  surmounted  by  platinum  points,  is  supported  by  a 
cylinder  of  glass  placed  under  the  dome  at  the  top  of  the  Ob» 
servatory.  The  copper  tube  passes  through  the  top  of  the  dome 
without  touching  it,  and  the  rain  is  excluded  from  this  opening 
by  an  inverted  copper  dish  fitted  to  the  tube.  This  copper  tube 
may  be  made  to  communicate  at  pleasure  with  the  electrometers. 

310.  Electrometers. — The  most  common  electrometer  is  Yolta's. 

This  consists  of  two  straws,  D,  Fig.  65,  two 
inches  in  length,  suspended  by  hooks  of  fine 
copper  wire,  and  at  a  distance  of  one  twen- 
tieth of  an  inch  from  each  other,  and  cover- 
ed by  a  glass  jar,  A.  When  the  two  straws 
are  similarly  electrified  they  recede  from 
each  other,  and  the  intensity  of  the  charge 
is  indicated  by  the  amount  of  the  divergence. 
This  divergence  is  measured  by  an  ivory 
scale  graduated  to  twentieths  of  an  inch. 
B  is  a  metallic  dish  to  protect  the  electron! 
eter  from  the  rain,  and  C  is  a  pointed  con- 
ductor for  collecting  the  electricity. 

It  is  desirable  to  have  a  series  of  electrom- 
eters for  measuring  electricity  of  different 
degrees  of  intensity.  For  the  feeblest  elec- 


Fig.  65. 


ELECTRICAL   PHENOMENA.  161 

ericity  the  gold-leaf  electrometer  may  sometimes  be  employed; 
and  when  the  electricity  is  very  intense  it  is  important  to  have 
an  instrument  for  measuring  the  length  of  the  spark.  This  may 
consist  of  a  sliding  rod  terminated  by  a  brass  ball,  which  can  be 
set  at  any  distance  from  the  insulated  conductor. 

311.  Electricity  at  considerable  Elevations. — The  electrical  con- 
dition of  the  higher  strata  of  the  air  has  been  ascertained  by 
means  of  kites  and  balloons.     When  a  kite  is  used  for  this  pur- 
pose, the  string  should  be  wound  with  fine  wire  in  order  to  make 
it  a  conductor  of  electricity,  and  the  kite  must  be  insulated  by 
attaching  the  lower  end  of  the  string  to  some  non-conductor  such 
as  silk  or  glass. 

Small  balloons  are  sometimes  employed  for  the  same  purpose, 
and  a  conducting  cord  connects  the  balloon  with  an  electrometer 
near  the  earth's  surface. 

By  instruments  like  these  it  is  found  that  the  air  is  generally 
charged  with  positive  electricity,  but  it  is  subject  to  great  varia- 
tions of  intensity,  and  clouds  are  frequently  charged  with  nega- 
tive electricity. 

312.  Diurnal  Variation  of  Electricity. — The  intensity  of  atmos- 
pheric electricity  is  found  to  vary  with  the  hour  of  the  day. 
From  the  mean  of  three  years'  observations  made  at  Kew,  it  ap- 
pears that  at  4  A.M.  the  electric  tension  is  represented  by  20  on 
Volta's  electrometer;  from  this  hour  the  electricity  increases  to 
10  A.M.,  when  it  is  represented  by  88;  from  that  time  it  de- 
creases to  4  P.M.,  when  it  is  represented  by  69 ;  it  then  increases 
to  10  P.M.,  when  it  is  represented  by  104;  from  which  time  it 
decreases  till  4  A.M.;  that  is,  there  are  two  daily  maxima  of  in- 
tensity and  two  daily  minima. 

313.  Monthly  Variation  of  Electricity. — The  intensity  of  atmos- 
pheric electricity  also  varies  with  the  season  of  the  year.     At 
Kew,  the  mean  electric  tension  is  least  in  June,  remaining  nearly 
the  same  through  the  summer  months,  after  which  the  electiv  ity 
increases  steadily  till  January,  continuing  nearly  the  same  through 
February,  after  which  it  decreases  till  the  next  June;  that  is,  there 
is  one  annual  maximum  of  intensity  and  one  minimum. 

At  Brussels,  also,  the  maximum  occurs  in  January  and  tha 

L 


162  METEOKOLOGY. 

minimum  in  June,  while  at  Munich  the  maximum  occurs  in  De- 
cember and  the  minimum  in  May. 

At  Brussels  the  electric  tension  in  winter  is  nine  times  as  great 
as  in  summer;  at  Kew  it  is  six  times  as  great;  and  at  Munich  it 
is  only  twice  as  great  in  winter  as  in  summer. 

314.  Variations  with  the  Altitude. — The  intensity  of  atmospher- 
ic electricity  increases  with  the  altitude  above  the  surface  of  the 
earth.     This  law  has  not  been  fully  verified  for  elevations  ex- 
ceeding 100  feet.     Experiments  with  electric  kites  have  obtained 
signs  of  electricity  the  more  powerful  as  the  kite  rose  to  a  great- 
er elevation.     Experiments  of  this  kind  have  been  carried  to  the 
height  of  810  feet. 

Similar  results  have  been  obtained  by  means  of  an  arrow 
projected  into  the  air,  the  arrow  being  provided  with  a  con- 
ducting wire  whose  extremity  communicated  with  a  straw  elec- 
trometer. 

Gay-Lussac,  during  his  aerial  voyage  in  1804,  suspended  from 
his  balloon  a  wire  170  feet  long,  and  connected  the  upper  end 
with  an  electrometer.  This  experiment  indicated  that  the  elec- 
tricity of  the  air  was  positive,  and  increased  with  the  altitude. 

In  a  balloon  ascent  in  1862,  Mr.  Glaisher  found  that  the  air 
was  charged  with  positive  electricity,  but  becoming  less  and  less 
in  amount  with  increasing  elevation,  till  at  the  height  of  23,000 
feet  the  amount  was  too  small  to  measure. 

315.  Electricity  in  cloudy  Weather. — When  the  sky  is  covered 
with  clouds,  the  electricity  is  subject  to  frequent  changes  of  kind 
as  well  as  intensity,  being  sometimes  positive   and  sometimes 
negative.     The  electricity  is  seldom  negative  except  when  T-ain 
is  falling.     During  a  snow-storm  the  lower  strata  of  the  air  ex 
hibit  electricity  of  great  intensity. 

During  the  passage  of  a  thunder -shower,  the  electricity  fre- 
quently changes  in  two  or  three  minutes  from  positive  to  nega- 
tive, and  then  back  again  to  positive,  sometimes  half  a  dozen  of 
these  changes  occurring  in  a  single  shower.  The  electricity  also 
at  such  times  has  great  intensity,  and  sparks  are  sometimes  ob- 
tained from  the  conductor  more  than  an  inch  in  length,  giving  a 
severe  shock  when  passed  through  the  human  system. 


ELECTRICAL   PHENOMENA.  103 

316.  Is  Atmospheric  Electricity  the  result  of  Friction? — Philoso- 
phers are  by  no  means  agreed  as  to  the  origin  of  atmospheric 
electricity.     Friction  is  one  of  the  most  common  sources  of  elec- 
tricity.    Dry  air  rubbing  against  dry  air,  or  any  other  substance, 
develops  little  if  any  electricity ;  but  moist  air  rubbing  against 
the  surface  of  the  earth  acquires  positive  electricity. 

In  violent  tornadoes  we  uniformly  observe  electricity  of  great 
intensity.  This  may  be  due  in  part  to  the  friction  of  the  air  upon 
the  earth.  But  we  can  not  consider  friction  to  be  the  principal 
source  of  atmospheric  electricity,  because  there  is  no  uniform 
relation  between  the  force  of  the  wind  and  the  intensity  of  the 
electricity. 

317.  Is  it  Hie  result  of  Combustion? — Combustion  is  another 
source  of  electricity.     When  coal  is  burning,  the  carbonic  acid 
gas  which  escapes  is  positively  electrified,  while  the  coal  has 
negative   electricity.      The   atmosphere,  therefore,  must  receive 
some  electricity  from  the  combustion  which  takes  place  on  the 
surface  of  the  earth ;  but  this  cause  must  be  entirely  inadequate 
to  account  for  the  enormous  quantities  of  electricity  exhibited  in 
thunder-showers. 

318.  Is  it  the  result  of  Vegetation? — Vegetation  is  a  source  of 
electricity.     During  the  day,  plants  give  out  oxygen  which  is 
charged  with  negative   electricity ;    and  during  the  night  they 
give  out  carbonic  acid  gas,  which  is  charged  with  positive  elec- 
tricity.    These  two  processes  in  a  measure  neutralize  each  other. 

319.  Is  it  the  result  of  unequal  Temperature? — The  unequal  tem- 
perature of  the  different  parts  of  the  earth  has  been  supposed  to 
be  a  source  of  atmospheric  electricity.     There  are  several  metals 
which  develop  electricity  when  brought  in  contact  and  unequal- 
ly heated.     In  some  of  the  mines  of  England,  currents  of  elec- 
tricity have  been  detected  within  the  earth,  and  these  currents 
have  been  ascribed  to  a  varying  temperature  acting  "upon  the 
heterogeneous  materials  of  the  earth. 

This  cause  may  explain  permanent  currents  existing  in  the 
earth,  but  does  not  seem  adequate  to  account  for  the  enormous 
quantity  of  free  electricity  which  often  exhibits  itself  in  thunder- 
showers. 


164  METEOROLOGY. 

320.  Is  it  the  result  of  sudden  Condensation  of  Vapor? — Since 
atmospheric  electricity  is  feeble  before  the  formation  of  a  storm, 
and  rapidly  attains  its  maximum  during  a  thunder-storm,  it  has 
been  supposed  that  electricity  is  liberated  in  the  act  of  condensa- 
tion of  the  vapor  of  the  air. 

When  the  steam  issuing  from  the  boiler  of  a  steam-engine  is 
suddenly  condensed,  a  great  amount  of  electricity  is  liberated. 
But  it  is  claimed  that  this  electricity  is  not  due  to  simple  con- 
densation, but  to  the  friction  of  the  condensed  particles  against 
the  sides  of  the  orifice  through  which  the  steam  escapes. 

321.  Is  Atmospheric  Electricity  due  to  Evaporation? — Evapora- 
tion is  probably  the  principal  source  of  atmospheric  electricity. 
The  following  experiment  shows  the  production  of  electricity  by 
evaporation.    If  upon  the  top  of  a  gold-leaf  electrometer  we  place 
a  metallic  vessel  containing  salt  water,  and  drop  into  the  water  a 
heated  pebble,  the  leaves  of  the  electrometer  will  diverge.     The 
vapor  which  rises  from  the  water  is  charged  with  positive  elec- 
tricity, while  the  water  retains  negative  electricity. 

The  water  used  in  this  experiment  must  not  be  perfectly  pure, 
but  must  contain  a  little  salt,  or  some  foreign  matter.  The  evap- 
oration of  the  water  of  the  ocean  must  therefore  furnish  a  large 
amount  of  electricity ;  and  fresh  water  must  also  furnish  some 
electricity,  for  the  water  of  the  earth  is  never  entirely  pure. 

322.  Diurnal  change  of  Electricity  explained. — The  diurnal  va- 
riation in  the  intensity  of  atmospheric  electricity  is  to  be  ascribed 
partly  to  real  changes  in  the  amount  of  electricity  present  in  the 
air,  and  partly  to  variations  in  the  conducting  power  of  the  air. 

Just  before  sunrise  the  electricity  has  a  feeble  intensity,  be- 
cause the  moisture  of  the  preceding  night  has  transmitted  to  the 
earth  a  portion  of  the  electricity  which  was  previously  present  in 
the  air.  After  the  sun  rises,  new  vapor  ascends  and  carries  with 
it  positive  electricity,  so  that  the  amount  of  electricity  in  the  air 
increases.  Toward  noon  the  air  becomes  dry,  and  transmits  less 
readily  the  electricity  accumulated  in  the  upper  regions  of  the 
atmosphere ;  so  that,  although  the  amount  of  electricity  in  the  air 
is  continually  increasing,  an  electrometer  near  the  earth's  surface 
indicates  an  apparent  diminution.  Toward  evening  the  air  grows 
cool,  again  becomes  humid,  and  transmits  more  readily  to  the 


ELECTRICAL   PHENOMENA.  165 

earth  the  electricity  accumulated  in  the  upper  regions  of  the  at- 
mosphere. The  effect  produced  upon  an  electrometer  therefore 
increases  until  some  hours  after  sunset;  but  since  during  the 
night  there  is  a  constant  discharge  of  electricity  from  the  air  to 
the  earth,  the  electrometer  soon  indicates  a  diminished  intensity, 
which  continues  until  toward  morning. 

323.  Monthly  change  of  Electricity  explained, — The  same  princi- 
ple explains  why  the  electricity  of  the  air  appears  less  intense  in, 
summer  than  in  winter.     In  summer  the  air  is  warm  and  dry, 
and  opposes  more  resistance  to  the  flow  of  electricity  from  the 
higher  regions  of  the  atmosphere,  while  in  winter  the  moist  air 
produces   a  contrary  effect ;    so  that,  although  the   atmosphere 
doubtless  contains  more  electricity  in  summer  than  in  winter,  it 
generally  produces  a  less  effect  upon  an  electrometer  placed  near 
the  earth's  surface. 

324.  Electricity  developed  in  dry  Houses.  —  During  the   cold 
weather  of  a  Northern  winter,  in  houses  which  are  kept  quite 
warm  and  dry,  and  whose  floors  are  covered  with  heavy  woolen 
carpets,  electricity  is  abundantly  excited  by  simply  walking  to 
and  fro  upon  the  carpet.     Sometimes  in  this  manner  there  is  de- 
veloped electricity  sufficient  to  give  an  unpleasant  shock,  and  to 
ignite  ether,  gas,  or  other  combustible  substances.     This  electrici- 
ty results  from  the  friction  of  dry  leather  upon  the  woolen  carpet, 
and  it  is  prevented  from  escaping  by  the  insulating  power  of  the 
dry  carpet,  and  the  extremely  dry  floor  of  the  building. 

SECTION   II 

THUNDER-STORMS. 

325.  How  clouds  become  Electrified. — We  have  found  that  the 
atmosphere  ordinarily  contains  a  large  quantity  of  electricity. 
Since  dry  air  is  a  non-conductor,  the  electrified  particles  in  clear 
weather  are  in  a  measure  insulated,  and  the  electricity  can  not 
acquire  much  intensity ;  but  when  the  vapor  of  the  air  is  precip- 
itated and  a  cloud  is  formed,  the  electricity,  which  was  previously 
confined  to  the  separate  particles  of  the  air,  now  finds  a  conduct- 
ing medium  more  or  less  perfect,  and  it  spreads  itself  over  the 
surface  of  the  cloud,  thereby  acquiring  considerable  intensity.    It 


166  METEOROLOGY. 

is  generally  admitted  that  the  same  quantity  of  electricity  which 
exists  in  the  cloud,  existed  in  the  air  before  the  formation  of  the 
cloud,  and  that  the  cloud  performs  no  other  office  than  that  of  a 
conductor. 

326.  Clouds  negatively  Electrified.  —  A  cloud  thus   electrified 
must  necessarily  have  positive  electricity,  since  in  clear  weather 
the  electricity  of  the  atmosphere  is  always  positive.      Such  a 
cloud,  when  it  approaches  near  another  cloud  having  less  elec- 
tricity, or  none  at  all,  acts  by  induction  upon  the  latter,  decom- 
posing its  natural  electricity,  attracting  the  negative  electricity 
and  repelling  the  positive.     The  positive  electricity  thus  repelled 
may  sometimes  be  drawn  off  by  near  approach  to  another  cloud, 
or  to  the  earth,  leaving  only  negative  electricity  upon  the  cloud. 
Hence  probably  result  the  frequent  alternations  of  positive  and 
negative  electricity  observed  during  a  thunder-shower. 

327.  Lightning. — Two  clouds  having  opposite  electricities  at- 
tract each  other,  and  when  the  clouds  come  sufficiently  near,  the 
two  electricities  rush  toward  each  other  with  great  violence.    This 
phenomenon  is  called  .lightning  ^  and  is  accompanied  by  an  explo- 
sive noise  called  thunder. 

Since  clouds  are  very  imperfect  conductors,  when  the  electricity 
of  one  part  of  a  cloud  is  discharged,  the  electricity  of  a  distant 
part  of  the  cloud  is  but  slightly  changed.  Thus  a  single  dis- 
charge does  not  establish  a  complete  electrical  equilibrium ;  but 
there  is  a  change  in  the  distribution  of  the  electricities  upon  the 
surrounding  clouds,  and  there  must  be  a  succession  of  discharges 
before  the  electricity  is  entirely  neutralized.  Hence  results  a  suc- 
cession of  flashes  of  lightning  and  peals  of  thunder. 

328.  Discharge  of  Electricity  to  the  Earth.  —  A  cloud  charged 
with  electricity  exerts  an  inductive  influence  upon  the  earth's 
surface  immediately  beneath  it,  decomposing  its  natural  electrici- 
ties, repelling  electricity  of  the  same  kind,  and  attracting  the  op- 
posite kind.     Accordingly  there  will  sometimes  be  a  discharge  of 
electricity  from  the  cloud  to  the  earth.     This  charge  is  usually 
received  by  the  most  elevated  objects,  such  as  mountains,  hills, 
trees,  spires,  high  buildings,  etc.     Trees  are  particularly  exposed 
to  strokes  of  lightning  on  account  of  their  elevation,  as  well  as  of 


ELECTRICAL   PHENOMENA.  167 

the  moisture  which  they  contain,  and  which  renders  them  partial 
conductors  of  the  electric  fluid. 

329.  Different  forms  of  Lightning. — Lightning  exhibits  a  variety 
of  forms,  which  have  been  designated  by  the  terms  zigzag,  ball, 
sheet,  and  heat  lightning. 

Zigzag  lightning  presents  a  long,  irregular,  jagged  line  of  light, 
like  the  ordinary  spark  drawn  from  an  electric  machine.  This 
zigzag  path  is  sometimes  four  or  five  miles,  and  perhaps  even  ten 
miles  in  length. 

The  irregularity  of  the  path  is  ascribed  to  the  compression  of 
the  air  before  the  electricity,  thereby  opposing  greater  resistance., 
and  turning  the  fluid  aside  to  seek  some  path  upon  which  the  re- 
sistance is  less. 

330.  Ball  Lightning. — Ball  lightning  appears  like  a  ball  of  fire, 
and  is  usually  accompanied  by  a  terrific  explosion.     It  probably 
results  from  a  charge  of  electricity  unusually  intense,  which  forces 
a  direct  instead  of  a  circuitous  passage  through  the  air. 

Some  have  supposed  that  ball  lightning  was  the  agglomera 
tion  of  ponderable  substances  in  a  state  of  great  tenuity,  strongly 
charged  with  electricity. 

331.  Sheet  Lightning. — Sheet  lightning  is  a  diffuse  glare  of  light, 
sometimes  illuminating  only  the  edges  of  a  cloud,  and  sometimes 
spreading  over  its  entire  surface. 

This  may  sometimes  be  due  to  distant  lightning  which  illu- 
mines a  cloud,  while  the  direct  flash  is  hidden  from  the  observer 
by  intervening  clouds.  Sometimes  it  may  result  from  a  move- 
ment of  electricity  in  the  interior  of  a  cloud  which  is  a  very  im 
perfect  conductor,  producing  an  illumination  analogous  to  that 
observed  on  a  plate  of  moist  glass  employed  in  discharging  an 
electrical  machine. 

332.  Heat  Lightning. — During  the  evenings  of  summer,  the 
horizon  is  sometimes  illumined  for  hours  in  succession  by  flashes 
of  light  unattended  by  thunder.     This  is  called  heat  lightning. 
This  illumination  is  sometimes  due  to  the  reflection  from  the  at- 
mosphere of  the  lightning  of  clouds  so  distant  that  the  thunder 
can  not  be  heard. 


168  METEOROLOGY. 

Sometimes,  however,  this  light  overspreads  the'entire  heavens, 
showing  that  the  electricity  of  the  clouds  escapes  in  flashes  so 
feeble  that  they  produce  no  audible  sound.  Such  cases  may  oc- 
cur when  the  air  is  very  moist,  the  air  being  then  a  tolerable  con- 
ductor, and  offering  just  sufficient  resistance  to  the  passage  of  the 
electricity  to  develop  a  feeble  light. 

333.  Color  of  Lightning. — The  color  of  lightning  varies  from 
white  to  a  rose  color  and  violet.     Zigzag  lightning  is  generally 
white,  sometimes  of  a  purplish  violet  or  bluish  tinge.     Diffuse 
flashes  of  lightning  are  often  of  an  intense  red,  sometimes  mixed 
with  blue  or  violet. 

These  differences  depend  upon  the  density  and  moisture  of  the 
strata  of  air  in  which  the  clouds  are  formed,  and  also  upon  its  con- 
ducting power.  When  the  density  of  the  medium  is  slight,  the 
light  becomes  diffuse  and  reddish  ;  when  the  density  is  considera- 
ble, the  light  is  concentrated  and  brilliant. 

Similar  variations  in  the  color  of  the  electricity  are  perceived 
when  the  fluid  is  passed  through  a  glass  receiver  in  which  the  air 
has  been  rarefied  by  means  of  an  air-pump. 

334.  Duration  of  Lightning. — The  duration  of  ordinary  flashes 
of  lightning  is  less  than  the  thousandth  part  of  a  second.     This  is 
proved  by  receiving  the  light  of  an   electric  discharge  upon  a 
white  disc  marked  with  black  rays,  when  the  disc  is  made  to  re- 
volve with  great  rapidity.     However  great  the  velocity  of  rota- 
tion may  be,  the  disc,  when  illumined  by  lightning,  always  appears 
stationary,  showing  that  during  the  continuance  of  the  illumina- 
tion the  disc  had  not  revolved  through  any  appreciable  angle. 
If  the  disc  were  illumined  for  an  instant  by  means  of  a  lamp,  by 
lifting  and  dropping  a  screen  as  suddenly  as  possible,  the  disc 
would  appear  of  a  uniform  tint,  and  no  separate  rays  would  be 
seen. 

335.  Cause  of  Thunder. — Thunder  is  generally  regarded  as  the 
result  of  the  sudden  re-entrance  of  the  air  into  a  void  space,  as  in 
the  experiment  of  a  bladder  tied  over  an  open-mouthed  receiver, 
and  burst  by  the  pressure  of  the  external  air.     This  vacuum  is 
supposed  to  be  generated  by  the  lightning  in  its  passage  through 
the  air:    Electricity  communicates  a  powerful  repulsive  force  to 


ELECTRICAL   PHENOMENA.  169 

the  particles  of  air  along  the  path  of  its  discharge,  producing  thus 
a  momentary  void,  into  which  immediately  afterward  the  sur- 
rounding air  rushes  with  a  violence  proportioned  to  the  intensity 
of  the  electricity. 

336.  Inlwval  between  the  Flash  and  Report. — Since  the  transmis- 
sion of  light  is  nearly  instantaneous,  while  sound  moves  only  1100 
feet  per  second,  the  sound  will  not  reach  the  ear  until  some  inter- 
val after  the  flash.     By  observing  the  interval  between  the  flash 
and  the  report,  the  distance  of  the  point  where  the  discharge  takes 
place  can  be  computed.     The  longest  interval  mentioned  by  any 
observer  is  72  seconds,  indicating  a  distance  of  15  miles.     With 
the  exception  of  this  single  instance,  the  longest  interval  recorded 
is  50  seconds,  indicating  a  distance  of  10  miles.     This  fact  is  very 
remarkable,  since  the  noise  of  cannon  may  be  heard  to  a  much 
greater  distance. 

The  average  interval  between  the  flash  and  the  report  is  12 
seconds,  and  the  shortest  interval  recorded  is  one  second. 

If  we  measure  the  angular  height  of  the  flash  whose  distance 
from  the  observer  has  been  determined,  we  may  compute  the  vert- 
ical elevation  of  the  cloud  above  the  earth. 

337.  Duration  of  Thunder. — Since  a  separate  sound  is  produced 

Fig. GO.  at  each  point  along  the  entire 

line  of  the  flash,  and  these 
points  are  generally  at  une- 
qual distances  from  the  ob- 
server, the  sounds  produced  at 
different  points  of  the  line  of 
discharge,  though  in  fact  si- 
multaneous, reach  the  ear  in 
slow  succession.  Thus  an  ob- 
server at  A,  Fig.  66,  will  first 
hear  the  sound  resulting  from 
the  concussion  at  a,  next  at  c,  and  finally  at  b.  If  b  were  11,000 
feet  more  remote  than  a,  the  first  sound  would  be  heard  ten  sec- 
onds before  the  last,  and  the  thunder  would  be  continuous  for 
ten  seconds. 

The  average  duration  of  peals  of  thunder  is  22  seconds,  and  the 
longest  duration  recorded  is  56  seconds. 


170  METEOEOLOGY. 

The  prolonged  duration  of  some  peals  of  thunder  is  in  part  the 
effect  of  echoes.  In  mountainous  countries  thunder  peals  are 
much  longer  continued,  and  the  sound  is  more  intense  than  in 
plane  countries.  This  is  due  to  the  reflection  of  the  thunder 
from  the  sides  of  the  mountains  in  the  same  manner  as  the  sound 
of  a  cannon  is  reflected.  These  echoes  may  also  be  produced  by 
reflection  of  sound  from  clouds,  as  has  been  proved  by  the  firing 
of  cannon  over  the  ocean. 

338.  Rolling  of  Thunder. — The  variable  intensity  or  rolling  of 
thunder  is  due  partly  to  the  zigzag  form  of  the  discharge,  in  con- 
sequence of  which  there  are  frequently  several  different  points  of 
the  flash  which  are  equally  distant  from  the  observer;  and  the 
sounds  produced  at  these  points  reach  the  ear  simultaneously, 
producing  the  effect  of  a  double  or  triple  sound. 

It  is  due  in  part  to  the  unequal  distance  of  different  parts  of 
the  flash,  the  loudness  of  sound  varying  inversely  as  the  square 
of  the  distance. 

It  may  also  be  due  m  part  to  the  fact  that  the  electricity,  in  its 
long  zigzag  course,  may  pass  through  strata  of  air  differing  ma- 
terially in  density,  which  may  result  either  from  difference  of  ele- 
vation or  difference  in  amount  of  moisture. 

The  rolling  of  thunder  is  also  without  doubt  in  a  considerable 
degree  the  effect  of  echoes. 

339.  Remarkable  succession  of  Phenomena  in  Thunder. — There  is 
a  certain  succession  of  phenomena  in  thunder  which  occurs  so 
frequently  as  to  indicate  that  ic  is  the  result  of  a  combination  of 
circumstances  of  common,  if  not  habitual  occurrence.    These  phe- 
nomena occur  in  the  following  order : 

1st.  The  flash  of  lightning. 

2d.  After  an  interval,  generally  of  10  or  12  seconds,  the  thunder 
begins  with  a  rattling  or  rumbling  noise,  which  increases,  some- 
times regularly,  sometimes  with  vibrations,  up  to  its  maximum. 

3d.  Five  or  ten  seconds  after  the  first  rumbling  we  hear  a  loud 
crashing  sound,  which  sometimes  continues  for  5,  10,  or  even  20 
seconds,  and  this  again  is  succeeded  by  a  rumbling  noise,  which 
gradually  dies  away. 

Sometimes  several  maxima  and  minima  succeed  each  other 
with  great  rapidity. 


ELECTRICAL   PHENOMENA.  171 

This  circumstance  of  a  crashing  sound  succeeding  by  a  consid- 
erable interval  the  first  rumbling  of  the  thunder  may  perhaps  be 
explained  by  the  imperfect  conducting  power  of  the  cloud.  If  we 
coat  a  Leyden  jar  with  brass  filings  instead  of  tin  foil,  and  charge 
it  with  electricity,  upon  discharging  the  jar  in  a  dark  room  we 
find  the  light  exhibits  numerous  ramifications,  spreading  out  like 
branches  from  the  trunk  of  a  tree.  A  similar  effect  may  be  pro- 
duced when  electricity  is  discharged  from  a  cloud.  Let  A  B,  Fig. 
67,  represent  the  zigzag  discharge  from  one  cloud  to  another,  and 

Fig.  67. 


suppose  the  discharge  of  electricity  from  the  interior  of  one  cloud 
takes  place  by  the  branches  AC,  AC', etc., and  from  the  interior 
of  the  other  cloud  by  the  branches  B  D,  B  D',  etc.  Then  an  ob- 
server at  E  would  first  hear  the  rattling  sound  resulting  from  the 
motion  of  the  electricity  along  the  paths  A  C,  A  C',  etc.,  and  this 
noise  would  not  be  of  very  great  intensity.  After  a  few  seconds 
the  sound  of  the  concentrated  discharge  through  A  B  will  reach 
him,  and  he  will  hear  a  crashing  noise,  which  will  continue  for 
several  seconds  with  variable  intensity.  This  will  be  succeeded 
by  a  low,  rumbling  noise,  resulting  from  the  partial  discharge 
along  B  D,  B  D',  etc.,  and  this  noise  will  be  faint  on  account  of  the 
great  distance. 

340.  Height  of  Thunder  Clouds.. —  Thunder  clouds  are  some- 
times limited  to  a  height  of  less  than  a  quarter  of  a  mile,  and 
sometimes  they  rise  to  the  height  of  at  least  three  or  four  miles. 
Observers  on  the  summit  of  hills  less  than  a  quarter  of  a  mile  in 
height,  have  seen  thunder-showers  below  them,  while  they  were 


172  METEOROLOGY. 

enjoying  a  cloudless  sky.  On  the  other  hand,  very  severe  thun- 
der-storms frequently  pass  over  the  summit  of  Pike's  Peak,  which 
is  elevated  14,150  feet  above  the  level  of  the  sea. 

341.  Lightning  Tubes. — When  lightning  descends  into  a  sandy 
soil,  the  sand  is  sometimes  melted  by  the  discharge,  and  the  path 
of  the  lightning  is  marked  by  a  tube  of  vitrefied  sand.     Such  a 
tube  is  called  a,  fulgurite.    These  tubes  are  sometimes  three  inches 
in  external  diameter,  with  sides  nearly  an  inch  in  thickness,  and 
they  sometimes  extend  to  a  depth  of  thirty  feet.     The  inside  part 
of  lightning  tubes  is  smooth  and  very  bright.     It  scratches  glass, 
and  strikes  fire  as  a  flint.     By  passing  a  powerful  electrical  dis- 
charge through  a  mixture  of  sand  and  salt,  similar  tubes  have 
been  produced  artificially. 

342.  Geographical  distribution  of  Thunder-storms. — Thunder- 
storms occur  most  frequently  in  the  equatorial  regions,  and  dimin- 
ish as  we  proceed  toward  the  poles.    From  the  equator  to  latitude 
30°  the  average  number  of  thunder-showers  annually  is  52  ;  from 
latitude  30°  to  latitude  50°  it  is  20 ;  from  latitude  50°  to  60°  it  is 
15 ;  and  from  latitude  60°  to  70°  it  is  only  4.     Beyond  latitude 
70°  lightning  is  of  very  rare  occurrence ;  and  beyond  the  parallel 
of  75°  it  is  believed  to  be  entirely  unknown. 

Within  the  tropics,  where  the  trade  winds  prevail,  thunder- 
storms are  rare ;  but  in  those  calm  regions  where  there  is  no 
steady  prevalent  wind  they  are  of  frequent  occurrence.  They 
are  produced  by  ascending  columns  of  air  in  the  form  of  torna- 
does; they  cover  but  a  small  area,  commence  suddenly,  and  rare- 
ly last  over  half  an  hour. 

In  Lower  Peru,  where  it  never  rains,  thunder  is  never  heard. 

Thunder-storms  are  most  frequent  in  warm  climates,  because 
here  evaporation  supplies  electricity  in  the  greatest  abundance, 
and  the  vapor  of  the  air  is  precipitated  most  copiously.  In  the 
middle  latitudes  thunder  occurs  chiefly  in  the  summer  months, 
and  it  is  most  frequent  about  the  middle  of  the  afternoon. 

343.  Lightning  caused  ly  Volcanoes. — The  eruptions  of  volca- 
noes are  frequently  accompanied  by  vivid  flashes  of  zigzag  light- 
ning.    This  electricity  is  probably  developed  in  the  same  way  as 
the  electricity  of  common  thunder-storms.     The  volcano  shoots 


ELECTRICAL   PHENOMENA.  173 

up  to  a  great  height  vast  volumes  of  heated  air.  This  air  is  cooled 
by  elevation,  its  vapor  is  condensed,  and  a  cloud  is  formed.  This 
cloud  serves  as  a  conductor  for  the  electricity  previously  existing 
in  the  air,  by  which  means  it  becomes  highly  charged,  and  the  elec- 
tricity thus  collected  is  discharged  upon  the  peak  of  the  volcano. 
For  the  same  reason,  violent  whirlwinds  and  water-spouts  are 
generally  attended  by  thunder  and  lightning. 

344.  Telegraph  Wires  affected  by  Thunder-storms. — The  wires  of 
the  electric  telegraph  present  conductors  of  electricity  of  vast  ex- 
tent, and  they  are  powerfully  affected  during  the  passage  of  a 
thunder-storm.     The  electricity  of  a  distant  cloud  is  sufficient  to 
charge  a  telegraph  wire,  and  when  the  electricity  of  the  cloud  is 
discharged,  a  spark  is  perceived  wherever  there  is  a  small  interrup- 
tion in  the  telegraph  wire.     This  effect  is  produced  at  a  distance 
of  several  miles,  and  during  summer  these  sparks  are  often  seen  in 
telegraph  offices,  being  sometimes  caused  by  a  thunder-storm  so 
remote  that  no  lightning  is  perceived  at  the  place  of  observation. 

345.  Pointed  Objects  tipped  with  Light. — If  in  a  dark  room  we 
hold  a  pointed  conductor  near  to  an  electrified  body,  we  may  ob- 
serve the  point  to  be  tipped  with  light.    Similar  phenomena  often 
occur  in  nature  upon  a  grand  scale.    When  the  lower  atmosphere 
is  highly  electrified,  pointed  objects  are  sometimes  seen  tipped 
with  light.     The  tops  of  the  masts,  and  the  ends  of  the  spars  of 
ships,  the  lances  of  soldiers,  the  tips  of  horses'  ears,  the  point  of 
an  umbrella,  and  similar  pointed  objects,  are  frequently  luminous 
at  night.     Sometimes  the  hair  of  the  head  stands  erect,  and  ap- 
pears tipped  with  flame. 

All  these  phenomena  are  due  to  a  moderate  charge  of  electrici- 
ty, not  sufficient  to  force  its  way  explosively,  but  escaping  by  a 
gentle  current. 

SECTION  III. 
AURORA    POLARIS. 

346.  The  aurora  polaris  is  a  luminous  appearance  frequently 
seen  near  the  horizon  as  a  diffuse  light  like  the  morning  twilight, 
whence  it  has  received  the  name  of  aurora.    In  the  northern  hem- 
isphere it  is  usually  termed  aurora  borealis,  because  it  is  chiefly 
seen  in  the  north.     A  similar  phenomenon  is  seen  in  the  south- 


174 


METEOROLOGY. 


ern  hemisphere,  where  it  is  called  the  aurora  australis.  Each  of 
them  may  with  greater  propriety  be  called  aurora  polaris,  or  po- 
lar light. 

347.  Varieties  of  Aurora. — Auroras  exhibit  an  infinite  variety 
of  appearances,  but  they  may  generally  be  referred  to  one  of  the 
following  classes: 

First.  A  horizontal  light  like  the  morning  aurora  or  break  of 
day.  The  polar  light  may  generally  be  distinguished  from  the 
true  dawn  by  its  position  in  the  heavens,  since  in  the  United 
States  it  always  appears  in  the  northern  quarter.  This  is  the 
most  common  form  of  aurora,  but  it  is  not  an  essentially  distinct 
variety,  being  due  to  a  blending  of  the  other  varieties  in  the  dis- 
tance. The  upper  limit  of  the  light  is  an  arc  of  a  small  circle, 
which,  though  indefinite,  is  better  defined  than  the  twilight. 

348.  Second.  An  Arch  of  Light  somewhat  in  the  form  of  a  Rain- 
bow.— This  arch  frequently  extends  entirely  across  the  heavens 
from  east  to  west,  and  cuts  the  magnetic  meridian  nearly  at  right 
angles.     This  arch  does  not  long  remain  stationary,  but  frequent- 
ly rises  and  falls ;  and  when  the  aurora  exhibits  great  splendor, 
several  parallel  arches  are  often  seen  at  the  same  time,  appearing 
as  broad  belts  of  light,  stretching  from  the  eastern  to  the  western 
horizon.      In  the  polar  regions,  five,  six,  and  even  seven  such 
arches  have  been  seen  at  once ;  and  on  two  occasions  have  been 
seen  nine  parallel  arches  separated  by  distinct  intervals.     Fig.  68 
represents  auroral  arches  seen  a  few  years  since  in  Canada. 

Fig.  08. 


ELECTRICAL   PHENOMENA. 


175 


349.  Third.  Slender,  luminous  beams  or  columns,  well  defined  and 
often  of  a  bright  light.  These  beams  rise  to  various  heights  in 
the  heavens  from  20°  or  30°  up  to  90°  or  more ;  sometimes,  though 
rarely,  passing  the  zenith,  Fig.  69.  Their  breadth  varies  from  a 


Fig.  69. 


quarter  of  a  degree  up  to  two  or  three  degrees.  Frequently  they 
last  but  a  few  minutes,  sometimes  they  continue  a  quarter  of  an 
hour,  a  half  hour,  or  even  a  whole  hour.  Sometimes  they  remain 
at  rest,  and  sometimes  they  have  a  quick  lateral  motion.  Their 
light  is  commonly  of  a  pale  yellow,  sometimes  reddish,  occasion- 
ally crimson,  or  even  of  blood  color.  Sometimes  the  luminous 
beams  are  interspersed  with  dark  rays  resembling  dense  smoke. 
Sometimes  the  tops  of  the  beams  are  pointed,  and,  having  a  wav- 
ing motion,  they  resemble  the  lambent  flames  of  half-extinguish' 
ed  alcohol  burning  upon  a  broad  flat  surface,  Fig.  70,  page  176. 
Faint  stars  are  visible  through  the  substance  of  the  beams. 

350.  Fourth.  The  Corona.— Luminous  beams  sometimes  shoot 
up  simultaneously  from  nearly  every  part  of  the  horizon,  and  con- 
verge to  a  point  a  little  south  of  the  zenith,  forming  a  quivering 
canopy  of  flame,  which  is  called  the  corona.  The  sky  now  resem- 
bles a  fiery  dome,  and  the  crown  appears  to  rest  upon  variegated 
fiery  pillars,  which  are  frequently  traversed  by  waves  or  flashes  of 
light.  This  may  be  called  a  complete  aurora,  and  comprehends 
most  of  the  peculiarities  of  the  other  varieties.  See  Fig.  74. 


176 
X" 


METEOROLOGY. 
Fig.  70. 


The  corona  seldom  continues  complete  longer  than  one  hour. 
The  streamers  then  become  fewer  and  less  intensely  colored ;  the 
luminous  arches  break  up,  while  a  dark  segment  is  still  visible 
near  the  northern  horizon,  and  at  last  nothing  remains  but  masses 
of  delicate  cirro-cumulus  clouds. 

During  the  exhibition  of  brilliant  auroras,  delicate  fibrous 
clouds  are  commonly  seen  floating  in  the  upper  regions  of  the 
atmosphere;  and  on  the  morning  after  a  great  nocturnal  display 
we  sometimes  recognize  the  same  streaks  of  cloud  which  had 
been  luminous  during  the  preceding  night.  Sometimes  during 
the  day  these  clouds  arrange  themselves  in  forms  similar  to  the 
beams  of  the  aurora,  constituting  what  has  been  called  a  day 
aurora. 

351.  Fifth.  Waves  or  Flashes  of  Light.  —  The  luminous  beams 
sometimes  appear  to  shake  with  a  tremulous  motion ;  flashes  like 
waves  of  light  roll  up  toward  the  zenith,  and  sometimes  travel 
along  the  line  of  an  auroral  arch.  Sometimes  the  beams  have  a 
slow  lateral  motion  from  east  to  west,  and  sometimes  from  west 
to  east.  These  sudden  flashes  of  auroral  light  are  known  by  the 
name  of  merry  dancers,  and  form  an  important  feature  of  nearly 
every  splendid  aurora. 


ELECTRICAL   PHENOMENA.  177 

352.  Duration  of  Auroras. —  The  duration  of  auroras  is  very 
variable.     Some  last  only  an  hour  or  two ;  others  last  all  night ; 
and  occasionally  they  appear  on  two  successive  nights  under  cir- 
cumstances which  lead  us  to  believe  that,  were  it  not  for  the  light 
of  the  sun,  an  aurora  might  be  seen  uninterruptedly  for  36  or  48 
hours.     For  more  than  a  week,  commencing  August  28th,  1859, 
in  the  northern  part  of  the  United  States,  the  aurora  was  seen  al- 
most uninterruptedly  every  clear  night.     In  the  neighborhood  of 
Hudson's  Bay,  the  aurora  is  seen  for  months  almost  without  ces- 
sation. 

353.  Recurring  Fits. — Auroras  are  characterized  by  recurring 
fits  of  brilliancy.     After  a  brilliant  aurora  has  faded  away,  and 
almost  wholly  disappeared,  it  is  common  for  it  to  revive,  so  as  to 
rival  and  often  to  surpass  its  first  magnificence.     Two  such  fits 
are  common  features  of  brilliant  auroras,  and  sometimes  three  or 
four  occur  on  the  same  night. 

354.  Colors  of  the  Aurora.  —  The  color  of  the  aurora  is  very 
variable.     If  the  aurora  be  faint,  its  light  is  usually  white  or  a 
pale  yellow.     When  the  aurora  is  brilliant,  the  sky  exhibits  at 
the  same  time  a  great  variety  of  tints ;  some  portions  of  the  sky 
are  nearly  white,  but  with  a  tinge  of  emerald  green ;  other  por- 
tions are  of  a  pale  yellow,  or  straw  color ;  others  are  tinged  with 
a  rosy  hue ;  while  others  have  a  crimson  hue,  which  sometimes 
deepens  to  a  blood  red.     These  colors  are  ever  varying  in  posi- 
tion and  intensity. 

355.  Geographical  Extent  of  Auroras. — Auroras  are  sometimes 
observed  simultaneously  over  large  portions  of  the  globe.     The 
aurora  of  August  28, 1859,  was  seen  over  more  than  140  degrees 
of  longitude,  from  California  to  Eastern  Europe,  and  from  Ja- 
maica, on  the  south,  to  an  unknown  distance  in  British  America, 
on  the  north.     The  aurora  of  September  2,  1859,  was  seen  at  the 
Sandwich  Islands ;  it  was  seen  throughout  the  whole  of  North 
America  and  Europe;  and  the  magnetic  disturbances  indicated 
its  presence  throughout  all  Northern  Asia,  although  the  sky 
was  overcast,  so  that  at  many  places  it  could  not  be  seen.     An 
aurora  was  seen  at  the  same  time  in  South  America  and  New 
Holland. 

M 


178 


METEOROLOGY. 


The  auroras  of  September  25, 1841,  and  November  17, 1848, 
were  almost  equally  extensive. 

356.  Dark  Segment. — In  the  United  States  an  aurora  is  uni- 
formly preceded  by  a  hazy  or  slaty  appearance  of  the  sky,  partic- 
ularly in  the  neighborhood  of  the  northern  horizon.  When  the 
auroral  display  commences,  this  hazy  portion  of  the  sky  assumes 
the  form  of  a  dark  bank  or  segment  of  a  circle  in  the  north,  rising 
ordinarily  to  the  height  of  from  five  to  ten  degrees,  Fig.  71.  This 


dark  segment  is  not  a  cloud,  for  the  stars  are  seen  through  it,  as 
through  a  smoky  atmosphere  with  little  diminution  of  brilliancy. 

This  dark  bank  is  simply  a  dense  haze,  and  it  appears  darker 
from  the  contrast  with  the  luminous  arc  which  rests  upon  it.  In 
high  northern  latitudes,  when  the  aurora  covers  the  entire  heav- 
ens, the  whole  sky  seems  filled  with  a  dense  haze ;  and  still  nearer 
the  pole,  where  the  aurora  is  sometimes  seen  in  the  south,  this 
dark  segment  is  observed  resting  on  the  southern  horizon,  and 
bordered  by  the  auroral  light.  This  phenomenon  was  visible  in 
the  United  States  in  the  aurora  of  August,  1859. 

The  highest  point  of  this  dark  segment  is  generally  found  in  the 
magnetic  meridian.  Exceptions,  however,  frequently  occur,  and 
in  some  places  there  is  a  constant  deviation  often  degrees  or  more. 


ELECTRICAL   PHENOMENA.  179 

357.  Position  of  Auroral  Arches. — The  dark  segment  is  bounded 
by  a  luminous  arc,  whose  breadth  varies  from  half  a  degree  to 
one  or  two  degrees.     The  lower  edge  is  well  defined ;  but,  unless 
the  breadth  be  very  small,  the  upper  edge  is  ill  defined,  and  blends 
with  a  general  brightness  of  the  sky.     If  the  aurora  becomes  bril- 
liant, other  arcs  usually  form  at  greater  elevations,  sometimes 
passing  through  the  zenith. 

The  summit  of  these  arcs  is  situated  nearly  in  the  magnetic 
meridian,  and  the  arc  sometimes  extends  symmetrically  on  each 
side  toward  the  horizon.  Frequently,  however,  the  summit  of 
the  arc  deviates  ten  degrees  or  more  from  the  magnetic  merid- 
ian, and  in  some  places  this  deviation  appears  to  be  tolerably  con- 
stant. 

Sometimes  the  arch  is  incomplete,  extending  only  part  of  the 
way  from  one  horizon  to  the  other. 

358.  Breadth  of  Auroral  Arches. — The  apparent  breadth  of 
auroral  arches  varies  with  their  elevation  above  the  horizon. 
The  result  of  a  large  number  of  observations  made  in  Scandina- 
via gave  seven  degrees  as  the  average  breadth  of  arches  seen  in 
the  north  at  altitudes  less  than  60° ;  for  arches  seen  in  the  south 
at  altitudes  less  than  60°,  the  average  breadth  was  eight  degrees ; 
while  for  arches  between  the  limits  of  30°  zenith  distance  either 
north  or  south,  the  average  breadth  was  twenty-five  degrees. 

When  an  arch  appears  to  move  across  the  sky  from  north  to 
south,  or  the  reverse,  its  angular  breadth  exhibits  corresponding 
changes. 

If  the  distance  of  an  arch  from  the  earth  remained  constant 
during  its  movement  of  translation,  and  the  arch  was  of  the  form 
of  a  ring  whose  section  was  a  circle,  its  breadth  when  in  the  ze- 
nith should  be  double  that  at  an  elevation  of  30°.  But  its  actual 
breadth  in  the  former  case  is  three  or  four  times  as  great  as  in  the 
latter,  showing  that  the  greatest  breadth  of  a  section  of  the  ring 
is  parallel  to  the  earth. 

359.  Form  of  Auroral  Arches. — Auroral  arches  are  not  arcs  of 
great  circles ;  that  is,  they  do  not  cut  the  horizon  at  points  180° 
from  each  other.     Careful  measurements  made  at  several  points 
of  some  of  the  most  remarkable  arcs  have  shown  that,  except 
near  the  horizon,  they  may  be  regarded  as  portions  of  small  cir* 


180  METEOROLOGY. 

oles  parallel  to  the  earth's  surface.  Such  a  circle  seen  obliquely 
would  have  the  appearance  of  an  ellipse.  Near  the  horizon  the 
elliptic  form  of  the  auroral  arch  has  sometimes  been  quite  notice- 
able, the  extremities  of  the  arch  being  bent  inward.  Occasion- 
ally an  ellipse  has  been  seen  almost  entire,  and  there  is  one  in- 
stance on  record  in  which  the  ellipse  appeared  complete,  the  di- 
ameters of  the  ellipse  being  as  two  to  one,  and  the  centre  of  the 
ellipse  being  elevated  about  15°  above  the  horizon. 

360.  Anomalous  forms  of  Arches. — Sometimes  an  auroral  arch 
consists  of  rays  arranged  in  irregular  and  sinuous  bands  of  vari- 
ous and  variable  curvatures,  presenting  the  appearance  of  the 
undulations  of  a  ribbon  or  flag  waving  in  the  breeze.  Some- 
times the  appearance  is  that  of  a  brilliant  curtain  whose  folds  are 
agitated  by  the  wind,  Fig.  72.  These  folds  sometimes  become 

Fig.T2. 


very  numerous  and  complex,  and  the  arch  assumes  the  form  of 
a  long  sheet  of  rays  returning  into  itself,  the  folds  enveloping 
each  other,  and  presenting  an  immense  variety  of  the  most  grace- 
ful curves.  Sometimes  these  curves  are  continually  changing, 
and  develop  themselves  like  the  folds  of  a  serpent. 

361.  Movements  of  Auroral  Arches. — An  auroral  arch  does  not 
maintain  invariably  a  fixed  position.  It  is  frequently  displaced, 
and  is  transported  parallel  to  itself  from  north  to  south,  or  from 


ELECTRICAL   PHENOMENA.  181 

i5outh  to  north.  An  arch  which  first  appears  near  the  northern 
horizon  sometimes  rises  gradually,  attains  the  zenith,  descends  to- 
ward the  southern  horizon,  remains  there  for  a  time  stationary, 
and  then  perhaps  retraces  its  course.  A  series  of  observations  in 
Scandinavia  presented  sixty  cases  in  which  auroral  arches  moved 
from  north  to  south,  and  thirty-nine  cases  from  south  to  north. 
In  the  United  States  the  motion  from  north  to  south  is  about  ten 
times  as  frequent  as  the  motion  from  south  to  north. 

Sometimes  there  is  a  movement  of  the  arch  from  west  to  east, 
or  from  east  to  west. 

The  rate  of  motion  of  arches  is  very  variable.  The  angular 
motion  of  translation  sometimes  amounts  to  17°  per  minute,  and 
frequently  amounts  to  5°  per  minute.  With  a  vertical  elevation 
of  125  miles  above  the  earth,  the  last  rate  of  motion  would  imply 
an  actual  velocity  of  1000  feet  per  second. 

362.  Structure  of  Auroral  Arches. — Auroral  arches  generally 
tend  to  divide  into  short  rays  running  in  the  direction  of  the 
breadth  of  the  arch,  and  converging  toward  the  magnetic  zenith. 
They  frequently  seem  to  be  formed  of  transverse  fibres  termin- 
ating abruptly  in  a  regular  curve,  which  forms  the  lower  edge  of 
the  arch,  Fig.  73.  Arches  entirely  nebulous  are  not  the  most  fre- 

Fig.  73. 


quent;  striated  arcs  are  very  common,  and  auroral  arches  present 
every  intermediate  variety  between  these  two  extremes.  Fre- 
quently a  nebulous  arc  resolves  itself  into  a  striated  arc  without 
changing  its  general  form.  Sometimes  the  rays  are  distinct  and 
isolated.  In  such  a  case  the  arch  generally  increases  in  breadth, 
extending  on  the  side  of  the  zenith.  Sometimes  auroral  beams 
arrange  themselves  in  the  form  of  an  arch,  which  is  subsequently 
replaced  by  an  arch  of  nebulous  matter.  When  the  light  of  the 


182  METEOROLOGY. 

rays  is  uniform,  the  dark  intervening  spaces  sometimes  present 
the  appearance  of  dark  rays  or  black  strice  perpendicular  to  the 
arch.  Sometimes  an  auroral  arch  is  formed  of  short  streams  par- 
allel to  each  other,  presenting  the  appearance  of  a  row  of  comet's 
tails. 

363.  Motion  of  Auroral  Seams. — This  motion  is  either  longitu- 
dinal, the  beam  extending  toward  the  zenith  or  the  horizon,  or  it 
is  a  lateral  movement  which  displaces  the  beam  parallel  to  itself. 
Frequently  a  beam  extends  suddenly  either  upward  or  down- 
ward.    This  motion  is  most  common  downward,  and  sometimes 
with  very  great  velocity.     It  sometimes  takes  place  simultane- 
ously in  a  large  number  of  neighboring  beams.     When  a  beam 
rises  and  falls  alternately  without  any  considerable  change  of 
length,  it  is  said  to  dance.     This  is  a  common  occurrence  in  high 
latitudes,  where  it  is  known  by  the  name  of  the  merry  dancers. 

Beams  sometimes  move  laterally  from  east  to  west,  and  some- 
times from  west  to  east ;  but  in  the  United  States  the  former  mo- 
tion is  the  most  common.  Beams  advance  either  from  north  to 
south,  or  from  south  to  north,  but  the  former  motion  is  the  most 
common. 

364.  The  Corona. — When  the  sky  is  filled  with  a  large  number 
of  separate  beams  all  parallel  to  each  other  and  to  the  direction 
of  the  dipping  needle,  according  to  the  rules  of  perspective,  these 
beams  will  seem  to  converge  to  one  point,  viz.,  the  magnetic  ze- 
nith, or  the  point  toward  which  the  dipping  needle  is  directed,  Fig. 
74.     Hence  results  the  appearance  of  a  corona,  or  crown  of  rays, 
whose  centre  is  generally,  but  not  always  dark. 

Numerous  measurements  have  been  made  of  the  position  of 
the  corona,  and  they  show  that  the  centre  of  the  corona  is  always 
very  near  the  magnetic  zenith,  but  not  always  exactly  coincident 
with  it. 

The  corona  is  sometimes  incomplete,  sectors  of  greater  or  less 
extent  being  deficient.  The  passage  of  a  striated  arch  over  the 
magnetic  zenith  frequently  presents  the  appearance  of  a  corona. 
If  the  arch  advances  from  north  to  south,  before  reaching  the 
magnetic  zenith  it  forms  a  half  crown  on  the  northern  side ;  at 
the  instant  of  passing  the  magnetic  zenith  we  have  a  complete" 
corona  of  an  elliptic  form,  whose  rays  descend  nearly  to  the  hori- 


ELECTRICAL   PHENOMENA. 

Fig.  T4. 


183 


zon  on  the  eastern  and  western  sides;  and  after  the  arch  has 
passed  the  magnetic  zenith  there  is  formed  a  half  crown  on  the 
southern  side. 

365.  Auroral  Clouds. — When  an  aurora  becomes  less  active  its 
beams  become  less  luminous,  their  edges  become  more  diffuse, 
they  increase  in  breadth  while  they  diminish  in  length,  and  as- 
sume the  appearance  of  luminous  clouds.     Sometimes  they  ex- 
hibit a  fibrous  structure,  and  present  a  strong  resemblance  to  cir- 
rus clouds.     These  auroral  clouds  generally  make  their  appear- 
ance later  in  the  evening  than  arches  or  beams. 

366.  Auroral  Vapor. — During  the  exhibition  of  a  brilliant  au- 
rora there  is  frequently  an  appearance  of  general  nebulosity  or 
luminous  vapor  covering  large  portions  of  the  heavens,  and  some- 


184 


METEOROLOGY. 


times  almost  the  entire  celestial  vault.  Its  light  is  generally  faint, 
especially  in  the  upper  part  of  the  sky,  sometimes  but  little  ex- 
ceeding that  of  the  milky  way  ;  but  sometimes,  near  the  horizon, 
the  light  is  intense,  resembling  a  vast  conflagration.  This  seems 
to  indicate  that  the  vertical  thickness  of  the  auroral  vapor  is  small 
in  comparison  with  its  horizontal  dimensions. 

This  auroral  vapor  may  appear  during  any  phase  of  a  grand 
aurora,  and  is  frequently  seen  during  the  intervals  between  the 
disappearance  and  reappearance  of  arches  and  beams. 

367.  Height  of  the  Aurora. — The  great  auroral  exhibition  of 
August  and  September,  1859,  was  very  carefully  observed  at  a 
large  number  of  stations,  and  these  observations  afford  the  ma- 
terials for  determining  the  height  of  the  aurora  above  the  earth's 
surface. 

At  the  most  southern  stations  where  these  auroras  were  ob- 
served, the  light  rose  only  a  few  degrees  above  the  northern 
horizon ;  at  more  northern  stations  the  aurora  rose  higher  in  the 
heavens;  at  certain  stations  it  just  attained  the  zenith  ;  at  stations 
farther  north  the  aurora  covered  the  entire  northern  heavens, 
as  well  as  a  portion  of  the  southern ;  and  at  places  farther  north 
nearly  the  entire  visible  heavens  from  the  northern  to  the  south- 
ern horizon  were  overspread  with  the  auroral  light. 

In  Fig.  75,  AB  represents  a  portion  of  the  earth's  surface,  and 
beneath  are  given  the  names  of  some  of  the  places  where  observ- 

Fig.  75. 


ations  were  made  upon  the  aurora  of  August  28, 1859,  all  at  the 
same  hour  of  the  evening.  The  dotted  lines  drawn  from  the 
five  most  southern  stations  represent  the  elevations  of  the  upper 


ELECTRICAL  PHENOMENA.  185 

boundary  of  the  auroral  light  above  the  northern  horizon.  The 
point  D  thus  determined  is  then  the  upper  edge  of  the  auroral 
light,  near  its  southern  margin,  and  this  point  is  found  to  be  534 
miles  above  the  earth's  surface. 

The  dotted  lines  from  the  five  most  northern  stations  show  the 
elevation  of  the  lower  limit  of  the  auroral  light  above  the  south 
horizon.  The  point  C  thus  determined  is  the  lower  edge  of  the 
auroral  light,  near  its  southern  margin,  and  this  point  is  found  to 
be  46  miles  above  the  earth's  surface.  The  line  CD  represents, 
therefore,  the  southern  boundary  of  the  auroral  illumination. 

These  results,  combined  with  a  vast  number  of  other  observ- 
ations, show  that  the  aurora  of  August  28th,  1859,  formed  a  stra- 
tum of  light  encircling  the  northern  hemisphere,  extending  south- 
ward in  North  America  to  latitude  38°,  and  reaching  to  an  un- 
known distance  on  the  north ;  and  it  pervaded  more  or  less  the 
entire  interval  between  the  elevations  of  46  miles  and  500  miles 
above  the  earth's  surface.  This  illumination  consisted  chiefly  of 
luminous  beams  or  columns  every  where  nearly  parallel  to  the  di- 
rection of  a  magnetic  needle  when  freely  suspended ;  that  is,  in 
the  United  States  the  upper  extremities  of  these  beams  inclined 
southward  at  angles  varying  from  15°  to  30°.  These  beams  were 
therefore  about  500  miles  in  length,  and  their  diameters  varied 
from  5  to  50  miles,  and  perhaps  sometimes  they  were  still  greater. 

The  height  of  a  large  number  of  auroras  has  been  computed 
by  similar  methods,  and  the  average  result  for  the  upper  limit  of 
the  streamers  is  450  miles. 

From  a  multitude  of  observations,  it  is  concluded  that  the  au- 
rora seldom  appears  at  an  elevation  less  than  about  45  miles 
above  the  earth's  surface,  and  that  it  frequently  extends  upward 
to  an  elevation  of  500  miles.  Auroral  arcs  having  a  well-defined 
border  are  generally  less  than  100  miles  in  height. 

368.  Conflicting  Estimates  of  the  Height. — Some  persons  contend 
that  the  aurora  is  sometimes  seen  at  elevations  of  less  than  one 
mile  above  the  earth's  surface.  It  is  claimed  that  the  aurora  is 
sometimes  seen  between  the  observer  and  a  cloud,  but  this  ap« 
pearance  is  believed  to  result  from  a  cloud  of  very  small  density 
strongly  illumined  by  auroral  light,  which  shines  through  the 
cloud  so  as  to  produce  the  same  appearance  as  if  the  aurora  pre« 
vailed  on  the  under  side  of  the  cloud. 


186  METEOROLOGY. 

Sometimes  the  lower  extremity  of  an  auroral  streamer  appears 
to  be  prolonged  below  the  summit  of  a  neighboring  mountain  or 
hill.  This  appearance  is  probably  an  illusion.  The  same  phe- 
nomenon has  been  noticed  by  careful  observers,  who  ascribed  the 
result  to  the  reflection  of  the  auroral  light  from  the  snow  which 
covered  the  mountains. 

Although  it  is  possible  that  the  aurora  may  sometimes  descend 
nearly  to  the  earth's  surface,  there  is  no  sufficient  evidence  to 
prove  that  the  true  polar  light  has  ever  descended  so  low  as  the 
region  of  ordinary  clouds. 

369.  Noise  of  the  Aurora.  —  There  is  no  satisfactory  evidence 
that  the  aurora  ever  emits  any  audible  sound.     It  is  a  common 
impression,  at  least  in  high  latitudes,  that  the  aurora  sometimes 
emits  sound.     This  sound  has  been  called  a   rustling,  hissing, 
crackling  noise.     But  the  most  competent  observers,  who  have 
spent  several  winters  in  the  Arctic  regions,  where  auroras  are  seen 
in  their  greatest  brilliancy,  have  been  convinced  that  this  sup- 
posed rustling  is  a  mere  illusion.    It  is  therefore  inferred  that  the 
sounds  which  have  been  ascribed  to  the  aurora  must  have  been 
due  to  other  causes,  such  as  the  motion  of  the  wind,  or  the  crack- 
ing of  the  snow  and  ice  in  consequence  of  their  low  temperature. 

If  the  aurora  emitted  an  audible  sound,  this  sound  ought  to 
follow  the  auroral  movements  after  a  long  interval.  Sound  re- 
quires four  minutes  to  travel  fifty  miles.  But  the  observers  who 
report  auroral  noises  make  no  mention  of  any  interval.  It  is 
therefore  inferred  that  the  sounds  which  have  been  heard  during 
auroral  exhibitions  are  to  be  ascribed  to  other  causes  than  the 
aurora. 

370.  Geographical  Distribution  of  Auroras.  —  Auroras  are  very 
unequally  distributed  over  the  earth's  surface.     They  occur  most 
frequently  in  the  higher  latitudes,  and  are  almost  unknown  with- 
in the  tropics.    At  Havana,  latitude  23°,  but  six  auroras  have  been 
recorded  within  a  hundred  years,  and  south  of  Havana  auroras 
are  still  more  unfrequent.     As  we  travel  northward  from  Cuba, 
auroras  increase  in  frequency  and  brilliancy ;  they  rise  higher  in 
the  heavens,  and  oftener  attain  the  zenith.     Near  the  parallel  of 
40°,  we  find,  on  an  average,  only  ten  auroras  annually.    Near  the 
parallel  of  42°,  the  average  number  is  twenty  annually;  near  45°, 


ELECTEICAL   PHENOMENA. 


187 


the  number  is  forty ;  and  near  the  parallel  of  50°,  it  amounts  to 
eighty  annually.  Between  this  point  and  the  parallel  of  62°, 
auroras  are  seen  almost  every  night.  They  appear  high  in  the 
heavens,  and  as  often  to  the  south  as  the  north.  Farther  north 
they  are  seldom  seen  except  in  the  south,  and  from  this  point 
they  diminish  in  frequency  and  brilliancy  as  we  advance  toward 
the  pole.  Beyond  latitude  62°  the  average  number  of  auroras  is 
reduced  to  forty  annually.  Beyond  latitude  67°  it  is  reduced  to 
twenty,  and  near  latitude  78°  to  ten  annually. 

Fig.  T6. 


188  METEOROLOGY. 

If  we  make  a  like  comparison  for  the  meridian  of  St.  Peters- 
burg, we  shall  find  a  similar  result,  except  that  the  auroral  region 
is  situated  farther  northward  than  it  is  in  America,  the  region  of 
eighty  auroras  annually  being  found  between  the  parallels  of  66° 
and  75°. 

Upon  Fig.  76,  the  dark  shade  indicates  the  region  where  the 
average  number  of  auroras  annually  amounts  to  at  least  eighty, 
and  the  lighter  shade  indicates  the  region  where  the  average 
number  of  auroras  annually  amounts  to  at  least  forty. 

We  thus  see  that  the  region  of  greatest  auroral  action  is  a  zone 
of  an  oval  form  surrounding  the  north  pole,  and  whose  central 
line  crosses  the  meridian  of  Washington  in  latitude  56°,  and  the 
meridian  of  St.  Petersburg  in  latitude  71°.  Accordingly,  auroras 
are  much  more  frequent  in  the  United  States  than  they  are  in  the 
same  latitudes  of  Europe. 

The  form  of  this  auroral  zone  bears  considerable  resemblance 
to  a  magnetic  parallel,  or  line  every  where  perpendicular  to  a 
magnetic  meridian,  and  it  is  probable  that  there  is  a  real  connec- 
tion between  the  two  phenomena. 

371.  Auroras  in  the  Southern  Hemisphere. — Auroras  in  the  south- 
ern hemisphere  are  nearly,  if  not  quite  as  frequent  as  they  are  in 
the  corresponding  magnetic  latitudes  of  the  northern  hemisphere, 
and  it  is  probable  that  the  geographical  distribution  of  auroras  in 
the  two  hemispheres  is  somewhat  similar. 

372.  Simultaneous  Auroras  in  both  Hemisplieres. — By  comparing 
the  records  of  auroras  in  the  two  hemispheres  we  find  a  remark- 
able coincidence  of  dates,  which  seems  to  justify  the  conclusion 
that  an  unusual  auroral  display  in  the  southern  hemisphere  is 
always  accompanied  by  an  unusual  display  in  the  northern  hem- 
isphere; that  is,  a  great  exhibition  of  auroral  light  about  one 
magnetic  pole  of  the  earth,  is  uniformly  attended  by  a  great  ex- 
hibition of  auroral  light  about  the  opposite  magnetic  pole. 

373.  Diurnal  Periodicity  of  Auroras.  —  Auroras  appear  at  all 
hours  of  the  night,  but  not  with  equal  frequency.     The  average 
number  increases   uninterruptedly  from  sunset  till  about  mid- 
night, from  which  time  the  number  diminishes  uninterruptedly 
till  morning.     In  Canada  the  maximum  occurs  an  hour  before 


ELECTRICAL  PHENOMENA.  189 

midnight ;  farther  north,  in  latitude  52°,  the  maximum  occurs  at 
midnight ;  and  still  farther  north  to  the  Arctic  Ocean,  the  maxi- 
mum occurs  an  hour  after  midnight. 

374.  Annual  Periodicity  of  Auroras. — Auroras  occur  in  each 
month  of  the  year,  but  not  with  equal  frequency.     In  New  En- 
gland and  New  York  the  least  number  of  auroras  is  recorded  in 
winter,  and  the  greatest  number  in  the  autumn ;  but  if  we  make 
allowance  for  the  diminished  length  of  the  nights  in  summer,  we 
must  conclude  that  auroras  are  about  as  frequent  in  summer  as 
in  autumn.     There  is  a  decided  diminution  in  the  frequency  of 
auroras  in  winter,  and  a  period  of  maximum  frequency  from 
April  to  September,  with  perhaps  a  slight  diminution  during  the 
intervening  month  of  June. 

Observations  in  Canada  lead  to  similar  conclusions,  except  that 
the  unequal  length  of  the  days  has  a  somewhat  greater  influence 
upon  the  number  of  auroras  recorded  in  summer, 

375.  Secular  Periodicity  of  Auroras. — In  attempting  to  decide 
whether  auroral  displays  exhibit  a  secular  periodicity,  some  dis- 
crimination is  required  in  selecting  the  data  for  comparison.     If 
for  each  year  we  employ  the  total  number  of  auroral  observations 
reported  from  all  parts  of  the  world,  the  results  exhibit  a  great 
inequality  on  those  years  for  which  we  have  reports  from  high 
northern  latitudes.     Moreover,  it  is  probable  that  in  the  high  lat- 
itudes the  inequality  of  auroral  displays  on  different  years  consists 
more  in  unequal  brilliancy  than  in  unequal  frequency  of  exhibi- 
tion ;  and  therefore  we  leave  out  of  account  all  the  occasional  ob- 
servations from  very  high  latitudes,  and  restrict  the  comparison 
to  a  region  of  the  globe  where  auroras  are  not  of  very  frequent 
occurrence.     For  this  purpose  we  draw  a  line  along  the  northern 
boundary  of  the  State  of  Massachusetts,  and  continue  thence  a 
line  of  equal  auroral  frequency  (Art.  370)  across  the  Atlantic  Ocean 
and  the  continent  of  Europe.     This  line  passes  near  the  cities  of 
Edinburgh  and  St.  Petersburg.     If  we  employ  all  the  auroras 
observed  at  stations  between  this  line  and  the  equator  within  the 
past  hundred  years,  the  resulting  numbers  clearly  indicate  that  in 
the  middle  latitudes  of  Europe  and  America  auroral  displays  ex- 
hibit a  true  periodicity,  and  these  periods  correspond  in  a  remark- 
able manner  with  those  shown  in  the  mean  daily  range  of  the 


190  METEOROLOGY. 

magnetic  declination,  and  in  the  extent  of  the  black  spots  on  the 
surface  of  the  sun.  The  grandest  displays  are  generally  repeated 
at  intervals  of  about  sixty  years;  and  there  are  other  fluctuations 
less  distinctly  marked  which  succeed  each  other  at  an  average 
interval  of  about  ten  or  eleven  years.  In  1870,  the  disturbance 
of  the  sun's  surface  was  the  greatest  which  has  ever  been  observed; 
the  fluctuations  of  the  magnetic  needle  were  uncommonly  great, 
and  auroral  displays  attained  a  magnificence  which  has  seldom 
been  equaled  in  the  middle  latitudes  of  the  northern  hemisphere. 
Since  1870,  there  has  been  a  rapid  decline  both  in  the  number 
of  the  solar  spots  and  in  the  frequency  of  auroral  exhibitions. 

376.  Disturbance  of  the  Magnetic  Needle. — The  aurora  is  ordina- 
rily accompanied  by  a  considerable  disturbance  of  the  magnetic 
needle,  and  the  effect  increases  with  the  brilliancy  and  extent  of 
the  aurora.     Auroral  beams  cause  a  disturbance  of  the  needle, 
particularly  when  the  beams  themselves  are  in  active  motion. 
Auroral  waves  or  flashes,  especially  if  they  extend  to  the  zenith, 
cause  a  violent  agitation  of  the  needle,  consisting  of  an  irregular 
oscillation  on  each  side  of  its  mean  position. 

These  extraordinary  deflections  of  the  needle  prevail  almost 
simultaneously  over  large  portions  of  the  globe,  even  where  the 
aurora  itself  is  not  visible.  During  the  great  auroral  display  of 
September  2, 1859,  the  disturbances  of  the  magnetic  needle  were 
very  remarkable  throughout  North  America,  Europe,  and  North- 
ern Asia,  as  well  as  in  New  Holland.  At  Toronto  the  declina- 
tion of  the  needle  changed  3°  45'  in  half  an  hour.  The  inclina- 
tion was  observed  to  change  2°  49'  when  the  needle  passed  be- 
yond the  limits  of  the  scale,  so  that  the  entire  range  of  the  needle 
could  not  be  determined.  The  horizontal  force  was  observed  to 
change  to  the  extent  of  one  ninth  of  its  whole  value  when  the 
needle  passed  beyond  the  limits  of  the  scale,  so  that  its  entire 
range  was  unknown.  At  several  observatories  in  Europe  still 
more  remarkable  disturbances  were  recorded. 

377.  Progress  of  Magnetic  Disturbances. — These  irregular  de- 
flections of  the  magnetic  needle  are  not  quite  simultaneous  at  dis- 
tant stations.     Over  the  surface  of  Europe  they  appear  to  be 
propagated  in  a  direction  from  N.  28°  E.  to  S.  28°  "W.  at  the  rate 
of  about  100  miles  per  minute.     Over  the  surface  of  North  Amer- 


ELECTRICAL  PHENOMENA.  191 

ica  they  are  propagated  at  about  the  same  velocity  in  a  direction 
from  N.  68°  E.  to  S.  68°  W. 

378.  Influence  of  the  Aurora  upon  the  Telegraph  Wires. — Auroras 
exert  a  remarkable  influence  upon  the  wires  of  the  electric  tele- 
graph.    During  the  prevalence  of  brilliant  auroras  the  telegraph 
lines  generally  become  unmanageable.    The  aurora  develops  elec- 
tric currents  upon  the  wires,  and  hence  results  a  motion  of  the 
telegraph  instruments  similar  to  that  which  is  employed  in  tele- 
graphing, and  this  movement,  being  frequent  and  irregular,  ordi- 
narily renders  it  impossible  to  transmit  intelligible  signals.    Dur- 
ing several  remarkable  auroras,  however,  the  currents  of  electrici- 
ty on  the  telegraph  wires  have  been  so  steady  and  powerful  that 
they  have  been  used  for  telegraph  purposes  as  a  substitute  for  a 
voltaic  battery ;  that  is,  telegraph  messages  have  been  transmitted 
from  the  auroral  influence  alone.     This  result  proves  that  the  au- 
rora develops  on  the  telegraph  wires  an  electric  current  similar 
to  that  of  a  voltaic  battery,  and  differing  only  in  its  variable  in- 
tensity. 

THEORY  OF  THE   POLAR  LIGHT. 

379.  Is  the  Aurora  caused  by  Nebulous  Matter  falling  into  our  At- 
mosphere?— Some  have  ascribed  the  polar  light  to  a  rare  nebulous 
matter  occupying  the  interplanetary  spaces,  and  revolving  round 
the  sun  at  such  a  distance  that  a  portion  of  this  matter  occasion- 
ally falls  into  the  upper  regions  of  the  atmosphere  with  a  velocity 
sufficient  to  render  it  luminous  from  the  condensation  of  the  air 
before  it.     But  we  can  see  no  reason  why  matter,  reaching  the 
earth  from  such  a  source,  should  always  be  confined  to  certain 
districts  of  the  earth,  and  be  wholly  unknown  in  other  portions. 
This  hypothesis,  therefore,  can  not  be  reconciled  with  the  known 
geographical  distribution  of  auroras. 

380.  Auroral  Exhibitions  are  Terrestrial  Phenomena. — Auroral 
exhibitions  take  place  in  the  upper  regions  of  the  atmosphere, 
and  partake  of  the  earth's  rotation.    All  the  celestial  bodies  have 
an  apparent  motion  from  east  to  west,  arising  from  the  rotation  of 
the  earth  ;  but  bodies  belonging  to  the  earth,  including  the  atmos- 
phere and  the  clouds  which  float  in  it,  partake  of  this  rotation,  so 
that  their  relative  position  is  not  affected  by  it.    The  same  is  true 


192  METEOROLOGY. 

of  the  aurora.  Whenever  a  corona  is  formed,  it  maintains  sensi- 
bly the  same  position  in  the  heavens  during  the  whole  period  of 
its  continuance,  although  the  stars  meanwhile  revolve  at  the  rate 
of  15°  per  hour. 

381.  The  Auroral  Light  is  Ekctric  Light  —  This  is  proved  by 
the  effect  of  an  aurora  upon  the  telegraph  wires.  The  electric 
telegraph  is  worked  by  a  current  of  electricity  generated  by  a 
voltaic  batteryr  and  flowing  along  the  conducting  wire  which 
unites  the  distant  stations.  This  current,  flowing  round  an  elec- 
tro-magnet, renders  it  temporarily  magnetic,  so  that  its  armature 
is  attracted,  and  a  mark  is  made  upon  a  roll  of  paper.  During  a 
thunder-storm  the  electricity  of  the  atmosphere  affects  the  con- 
ducting wire  in  a  similar  manner,  and  a  great  auroral  display 
produces  a  like  effect.  During  the  auroras  of  August  and  Sep- 
tember, 1859,  there  were  remarked  all  those  classes  of  effects 
which  are  considered  as  characteristic  of  electricity. 

A.  In  passing  from  one  conductor  to  another,  electricity  exhib- 
its a  spark  of  light.     During  the  auroras  of  1859,  at  numerous 
stations  both  in  America  and  Europe,  brilliant  sparks  were  drawn 
from  the  telegraph  wires  when  no  battery  was  attached. 

B.  In  passing  through  poor  conductors,  electricity   develops 
heat.     During  the  auroras  of  1859,  both  in  America  and  Europe, 
paper  and  even  wood  were  set  on  fire  by  the  auroral  influence 
alone. 

C.  When  passed  through  the  animal  system,  electricity  com- 
municates a  well-known  characteristic  shock.     During  the  auroras 
of  1859  several  telegraph  operators  received  severe  shocks  when 
they  touched  the  telegraph  wires. 

D.  A  current  of  electricity  develops  magnetism  in  ferruginous 
bodies.     The  auroras  of  1859  developed  magnetism  so  abundant- 
ly and  so  steadily  that  it  was  more  than  sufficient  for  the  ordi- 
nary business  of  telegraphing. 

E.  A  current  of  electricity  deflects  a  magnetic  needle  from  its 
normal  position.     In  England  the  usual  telegraph  signal  is  made 
by  a  magnetic  needle  surrounded  by  a  coil  of  copper  wire,  so 
that  the  needle  is  deflected  by  an  electric  current  flowing  through 
the  wire.    Similar  deflections  were  caused  by  the  auroras  of  1859, 
and  these  deflections  were  greater  than  those  produced  by  the 
telegraph  batteries. 


ELECTRICAL  PHENOMENA.  198 

F.  A  current  of  electricity  produces  chemical  decompositions, 
The  auroras  of  1859  produced  the  same  marks  upon  chemical  pa- 
per as  are  produced  by  an  ordinary  voltaic  battery ;  that  is,  they 
decomposed  a  chemical  compound. 

Gr.  Certain  bodies,  such  as  a  solution  of  sulphate  of  quinine, 
when  illumined  by  an  electric  spark,  present  a  very  peculiar  ap- 
pearance, as  if  they  were  self-luminous.  This  appearance  is 
termed  fluorescence.  The  same  effect  is  produced  upon  these  sub- 
stances by  the  auroral  light. 

These  facts  demonstrate  that  the  fluid  developed  by  the  aurora 
on  telegraph  wires  is  indeed  electricity.  This  electricity  may  be 
derived  from  the  aurora  either  by  transfer  or  by  induction.  If 
we  adopt  the  former  supposition,  then  the  auroral  light  is  certain- 
ly electric  light.  If  we  adopt  the  latter  supposition,  then,  since 
we  know  of  but  two  agents,  magnetism  and  electricity,  capable  of 
inducing  electricity  in  a  distant  conductor,  and  since  the  auroral 
fluid  is  luminous  while  magnetism  is  not  luminous,  we  must  ad- 
mit that  the  auroral  light  is  electric  light. 

382.  Colors  of  the  Aurora.  —  The  colors  of  the  aurora  are  the 
same  as  those  of  ordinary  electricity  passed  through  rarefied  air. 
When  a  spark  is  drawn  from  an  ordinary  electrical  machine  in 
air  of  the  usual  density,  the  light  is  intense  and  nearly  white.    If 
the  electricity  be  passed  through  a  glass  vessel  in  which  the  air 
has  been  partially  rarefied,  the  light  is  more  diffuse,  and  inclines 
to  a  delicate  rosy  hue.    If  the  air  be  still  farther  rarefied,  the  light 
becomes  very  diffuse,  and  its  color  becomes  a  deep  rose  or  purple. 
The  same  variety  of  colors  is  observed  during  the  aurora.     The 
transition  from  a  white  or  pale  straw  color  to  a  rosy  hue,  and 
finally  to  a  deep  red,  probably  depends  upon  the  height  above  the 
earth,  and  upon  the  amount  of  condensed  vapor  present  in  the  air. 

The  emerald  green  light  which  is  seen  in  some  auroras  is  as- 
cribed to  the  projection  of  the  yellow  light  of  the  aurora  upon 
the  blue  sky,  since  a  combination  of  yellow  and  blue  light  pro- 
duces green.  A  similar  effect  is  often  produced  in  the  evening 
twilight  by  a  combination  of  the  yellow  light  of  the  sun  with  the 
blue  of  the  celestial  vault. 

383.  The  Auroral  Corona. — The  formation  of  an  auroral  corona 
near  the  magnetic  zenith  is  the  effect  of  perspective,  resulting 

N 


194  METEOROLOGY. 

from  a  great  number  of  luminous  beams  all  parallel  to  each  other. 
A  collection  of  beams  parallel  to  the  direction  of  the  dipping 
needle  would  all  appear  to  converge  toward  the  pole  of  the  nee- 
dle, and  no  other  supposition  will  explain  all  the  appearances 
which  we  observe.  The  auroral  crown,  therefore,  every  where 
appears  in  the  magnetic  zenith,  and  it  is  not  the  same  crown 
which  is  seen  at  different  places  any  more  than  it  is  the  same 
rainbow  which  is  seen  by  different  observers. 

384.  What  are  Auroral  Beams? — The  auroral  beams  are  sim- 
ply illumined  spaces  caused  by  the  flow  of  electricity  through  the 
upper  regions  of  the  atmosphere.     During  the  auroras  of  1859 
these  beams  were  nearly  500  miles  in  length,  and  their  lower  ex- 
tremities were  elevated  about  45  miles  above  the  earth's  surface. 
Their  tops  inclined  toward  the  south  about  17°  in  the  neighbor- 
hood of  New  York. 

It  was  formerly  supposed  that  the  electric  current  necessarily 
moved  in  the  direction  of  the  axis  of  the  auroral  beams;  that  is, 
that  the  electric  discharge  was  from  the  upper  regions  of  the  at- 
mosphere to  the  earth,  or  the  reverse.  Recent  discoveries  throw 
some  doubt  upon  this  conclusion.  When  a  stream  of  electricity 
flows  through  a  vessel  from  which  the  air  is  almost  wholly  ex- 
hausted, under  certain  circumstances  the  light  becomes  stratified, 
exhibiting  alternately  bright  and  dark  bands  crossing  the  electric 
current  at  right  angles,  from  which  it  might  be  inferred  that  elec- 
tricity flowing  horizontally  through  the  upper  regions  of  the  at- 
mosphere might  exhibit  alternately  bright  and  dark  bands  like 
the  auroral  beams.  But  this  stratification  of  the  electric  light  is 
due  to  intermittences  in  the  intensity  of  the  electric  discharge,  and 
it  is  not  probable  that  such  intermittences  can  take  place  in  na- 
ture with  sufficient  rapidity  to  produce  a  similar  effect.  It  is  there- 
fore more  probable  that  auroral  beams  are  the  result  of  a  current 
of  electricity  traveling  in  the  direction  of  the  axis  of  the  beams. 

385.  Cause  of  the  Dark  Segment. — The  slaty  appearance  of  the 
sky  which  is  remarked  in  all  great  auroral  exhibitions  arises  from 
the  condensation  of  the  vapor  of  the  air,  and  this  condensed  va- 
por probably  exists  in  the  form  of  minute  spiculre  of  ice  or  flakes 
of  snow.     Fine  flakes  of  snow  have  been  repeatedly  observed  to 
fall  during  the  exhibition  of  auroras,  and  this  snow  only  slightly 


ELECTRICAL   PHENOMENA.  195 

impairs  the  transparency  of  the  atmosphere,  without  presenting 
the  appearance  of  clouds.  It  produces  a  turbid  appearance  of  the 
atmosphere,  and  causes  that  dark  bank  which  in  the  United  States 
rests  on  the  northern  horizon.  This  turbidness  is  more  noticea- 
ble near  the  horizon  than  it  is  at  great  elevations,  because  near 
the  horizon  the  line  of  vision  traverses  a  greater  depth  of  this 
hazy  atmosphere.  When  the  aurora  covers  the  whole  heavens, 
the  entire  atmosphere  is  filled  with  this  haze,  and  a  dark  seg- 
ment may  be  observed  resting  on  the  southern  horizon. 

386.  Circulation  of  Electricity  about  the  Earth. — The  vapor  which 
rises  from  the  ocean  in  all  latitudes,  but  most  abundantly  in  the 
equatorial  regions  of  the  earth,  carries  into  the  upper  regions  of 
the  atmosphere  a  considerable  quantity  of  positive  electricity, 
while  the  negative  electricity  remains  in  the  earth.  This  posi- 
tive electricity,  after  rising  nearly  vertically  with  the  ascending 
currents  of  the  atmosphere,  would  be  conveyed  toward  either 
pole  by  the  upper  currents  of  the  atmosphere. 

The  earth  and  the  rarefied  air  of  the  upper  atmosphere  may  be 
regarded  as  forming  the  two  conducting  plates  of  a  condenser, 
which  are  separated  by  an  insulating  stratum,  viz.,  the  lower  por- 
tion of  the  atmosphere.  The  two  opposite  electricities  must  then 
be  condensed  by  their  mutual  influence,  especially  in  the  polar 
regions,  where  they  approach  nearest  together,  and  whenever  their 
tension  reaches  a  certain  limit,  there  will  be  discharges  from  one 
conductor  to  the  other.  When  the  air  is  humid  it  becomes  a 
partial  conductor,  and  conveys  a  portion  of  the  electricity  of  the 
atmosphere  to  the  earth.  On  account  of  the  low  conducting  pow- 
er of  the  air,  the  neutralization  of  the  opposite  electricities  would 

not  be  effected  instantaneouslv,  but 

• ' 

by  successive  discharges,  more  or 
less  continuous,  and  variable  in  in- 
tensity. These  discharges  should 
frequently  occur  simultaneously  at 
the  two  poles,  since  the  electric  ten- 
sion of  the  earth  should  be  nearly  J 
the  same  at  each  pole. 

Fig.  77  represents  the  system  of 
circulation  here  supposed,  the  north 
and  south  poles  of  the  earth  being 
denoted  bv  the  letters  N.  and  S. 


196  METEOROLOGY. 

387.  Cause  of  the  Auroral  Beams. — When  electricity  from  the 
upper  regions  of  the  atmosphere  discharges  itself  to  the  earth 
through  an  imperfectly  conducting  medium,  the  flow  can  not  be 
every  where  uniform,  but  must  take  place  chiefly  along  certain 
lines  where  the  resistance  is  least ;  and  this  current  must  develop 
light,  forming  thus  an  auroral  beam.     It  might  be  expected  that 
these  beams  would  have  a  vertical  position,  but  their  position  is 
controlled  by  the  earth's  magnetism.     It  is  found  that  when  mag- 
netic forces  act  upon  a  perfectly  flexible  conductor  through  which 
an  electric  current  passes,  the  conductor  must  assume  the  form  of 
a  magnetic  curve.     Now  at  each  point  of  the  earth's  surface  the 
dipping  needle  shows  the  direction  of  the  magnetic  curve  passing 
through  that  point.     Hence  the  axis  of  an  auroral  streamer  must 
lie  in  the  magnetic  curve  which  passes  through  its  base;  and  since 
adjacent  streamers  are  sensibly  parallel,  the  beams  appear  to  con- 
verge toward  the  magnetic  zenith. 

388.  Position  of  Auroral  Arches  explained. — When  electricity 
escapes  from  a  metallic  conductor  under  a  receiver  from  which 
the  air  has  been  exhausted,  and  this  conductor  is  the  pole  of  a 
powerful  magnet,  the  electric  light  forms  a  complete  luminous 
ring  around  this  conductor. 

In  like  manner,  the  auroral  arch  is  a  part  of  a  luminous  ring 
nearly  parallel  to  the  earth's  surface,  having  the  magnetic  pole  for 
its  centre,  and  cutting  all  the  magnetic  meridians  at  right  angles ; 
and  this  position  results  from  the  influence  of  the  earth's  mag- 
netism. 

389.  Anomalous  Position  of  Auroral  Arches. — Auroral  arches  are 
not  always  exactly  perpendicular  to  the  magnetic  meridian,  and 
in  some  places  this  deviation  is  uniform,  and  may  amount  to  ten 
degrees.     Such  a  deviation  may  be  explained  as  follows : 

The  direction  of  the  magnetic  needle  at  any  place  is  determ- 
ined mainly  by  its  position  with  respect  to  the  magnetic  poles  of 
the  earth,  but  partly  by  local  causes,  such  as  the  conformation  of 
the  land  and  sea,  etc.  In  consequence  of  these  local  causes,  the 
direction  of  the  magnetic  needle  at  some  places  probably  differs 
several  degrees  from  what  it  would  be  if  it  were  controlled  en- 
tirely by  the  magnetic  poles.  This  local  influence  probably  di- 
minishes as  we  rise  above  the  earth's  surface,  so  that  at  the  height 


ELECTRICAL   PHENOMENA.  197 

of  the  auroral  streamers  the  direction  of  the  magnetic  needle  may 
differ  several  degrees  from  that  at  the  surface  of  the  earth. 

390.  Cause  of  the  Auroral  Flashes. — The  flashes  of  light  ob- 
served in  great  auroral  displays  are  due  to  inequalities  in  the  mo- 
tion of  the  electric  currents.     On  account  of  the  imperfect  con- 
ducting power  of  the  air,  the  flow  of  electricity  is  not  perfectly 
uniform,  but  escapes  by  paroxysms.     The  flashes  of  the  aurora 
are  therefore  feeble  flashes  of  lightning. 

391.  Cause  of  the  Magnetic  Disturbances. — The  disturbance  of 
the  magnetic  needle  during  auroras  is  due  to  currents  of  electric- 
ity flowing  through  the  atmosphere  or  through  the  earth.     A 
magnetic  needle  is  deflected  from  its  mean  position  by  an  electric 
current  flowing  near  it  through  a  good  conductor  like  a  copper 
wire.     A  stream  of  electricity  flowing  through  the  earth  or  the 
atmosphere  must  produce  a  similar  effect. 

It  is  probable  that  the  directive  power  ot  the  magnetic  needle 
is  due  to  electric  currents  circulating  around  the  globe  from  east 
to  west.  Such  currents  would  cause  the  magnetic  needle  every 
where  to  assume  a  position  corresponding  with  what  is  actually 
observed ;  and  the  existence  of  such  currents  has  been  proved  by 
direct  observation. 

According  to  Art.  386,  positive  electricity  circulates  from  the 
equator  toward  either  pole  through  the  upper  regions  of  the  at- 
mosphere, and  thence  thrpugh  the  earth  toward  the  equator,  to  re- 
store the  equilibrium  which  is  continually  disturbed  by  evapora- 
tion from  the  waters  of  the  equatorial  seas.  This  current  from 
the  polar  regions  must  modify  the  regular  current  which  is  sup- 
posed to  be  constantly  circulating  from  east  to  west,  resulting  in 
a  current  from  northeast  to  southwest,  in  conformity  with  observ- 
ations. 

This  current  does  not,  however,  flow  uninterruptedly  from  N".E. 
to  S.W.,  but  alternates  at  short  intervals  with  a  current  in  the 
contrary  direction.  Such  currents  of  electricity  must  produce  a 
continual  disturbance  of  the  magnetic  needle,  and  they  are  suffi- 
cient to  account  for  the  disturbances  actually  observed. 

392.  Effect  of  the  Aurora  upon  Telegraph  Wires. — The  effect  of 
the  aurora  upon  the  telegraph  wires  is  similar  to  that  of  electric- 


198  METEOKOLOGY. 

ity  in  thunder-storms,  except  in  the  intensity  and  steadiness  of  its 
action.  During  thunder-storms  the  electricity  of  the  wires  is  dis- 
charged instantly  with  a  flash  of  lightning,  while  during  auroras 
there  is  sometimes  a  strong  and  steady  flow  continuing  for  several 
minutes. 

393.  Cause  of  the  Diurnal  Inequality  of  Auroras. — The  diurnal 
inequality  in  the  frequency  of  auroras  is  due  to  the  same  cause  as 
the  diurnal  variation  in  the  intensity  of  atmospheric  electricity. 
The  same  causes  which  favor  the  escape  of  electricity  from  the 
upper  atmosphere  to  the  earth  will  produce  an  aurora  whenever 
the  electricity  of  the  upper  air  is  sufficiently  intense,  and  the  con- 
ducting power  of  the  air  is  favorable  for  the  slow  transmission  of 
an  electric  current. 

394.  Cause  of  the  Annual  Inequality  of  Auroras. — The  unequal 
frequency  of  auroras  in  the  different  months  of  the  year  depends 
partly  upon  the  amount  of  electricity  present  in  the  upper  air, 
and  partly  upon  the  humidity  of  the  air  by  which  this  electricity 
may  be  discharged.     The  supply  of  electricity  must  be  greatest 
when  the  evaporation  is  most  rapid,  that  is,  in  summer,  and  this  is 
probably  the  reason  why  in  North  America  auroras  are  more  fre- 
quent in  summer  than  in  winter.     In  Europe  auroras  are  seldom 
seen  in  midsummer,  because  in  those  latitudes  where  auroras  are 
most  frequent,  twilight  in  midsummer  continues  all  night. 

395.  Cause  of  the  Secular  Inequality  of  Auroras. — The  secular 
inequality  in  the  frequency  of  auroras  indicates  the  influence  of 
distant  celestial  bodies  upon  the  electricity  of  our  globe.     The 
periods  of  auroras  observe  laws  which  are  similar  to  those  of  two 
other  phenomena,  viz.,  the  mean  diurnal  variation  of  the  magnetic 
needle,  and  the  frequency  of  black  spots  upon  the  sun's  surface. 

The  magnetic  needle  has  a  small  diurnal  variation,  the  north 
end  moving  a  little  to  the  east  in  the  morning,  and  toward  the 
west  about  the  middle  of  the  day.  The  mean  daily  change  of  the 
needle  not  only  varies  with  the  locality,  but  also  varies  from  one 
year  to  another  at  the  same  locality,  and  these  variations  present 
a  decided  appearance  of  periodicity.  At  Prague  the  mean  daily 
change  of  the  needle  in  1838  was  12',  from  which  time  the  range 
diminished  steadily  to  1844,  when  it  was  only  6',  from  which  time 


ELECTRICAL   PHENOMENA.  199 

it  increased  to  1848,  when  it  amounted  to  11',  the  interval  from 
one  maximum  to  another  being  a  little  more  than  ten  years. 

Observations  made  at  other  places,  and  extending  back  nearly 
a  century,  indicate  a  maximum  in  the  range  of  the  magnetic  nee- 
dle every  ten  or  eleven  years,  but  the  successive  maxima  are  not 
equal  to  each  other.  They  exhibit  variations  which  indicate  a 
periodicity,  the  greatest  values  occurring  at  intervals  of  from  fifty 
to  sixty  years.  See  Table  XXXIV. 

The  relative  frequency  of  the  solar  spots  exhibits  a  similar  pe- 
riodicity, and  the  maximum  number  of  spots  corresponds  with 
the  maximum  value  of  the  magnetic  variation. 

These  three  phenomena,  the  solar  spots,  the  mean  daily  range 
of  the  magnetic  needle,  and  the  frequency  of  auroras,  exhibit  two 
distinct  periods;  one  a  period  of  from  ten  to  twelve  years,  the 
other  a  period  of  from  fifty-eight  to  sixty  years.  The  first  of 
these  periods  corresponds  to  one  revolution  of  Jupiter,  and  the 
other  period  corresponds  to  five  revolutions  of  Jupiter,  or  two  of 
Saturn,  and  we  can  scarcely  doubt  that  the  above-mentioned  phe- 
nomena depend  upon  the  movements  of  these  planets.  Observa- 
tions have  also  indicated  subordinate  fluctuations  which  are  prob- 
ably due  to  the  action  of  Venus. 

We  do  not  know  how  the  planets  exert  an  influence  upon  the 
sun's  surface ;  but  we  may  suppose  that  there  are  circulating 
round  the  sun  powerful  electric  currents,  which  may  possibly  be 
the  source  of  the  sun's  light;  these  currents  may  act  upon  the 
planets,  developing  in  them  electric  currents;  and  the  currents 
circulating  round  the  planets  may  react  upon  the  solar  currents 
with  a  force  varying  with  their  distances  and  relative  positions, 
exhibiting  periods  corresponding  to  the  times  of  revolution  of 
the  planets.  These  disturbances  of  the  solar  currents  may  be 
one  cause  of  the  solar  spots,  and  an  unusual  disturbance  of  the 
solar  currents  may  cause  a  disturbance  of  the  currents  of  the 
earth's  surface,  giving  rise  to  unusual  displays  of  the  aurora. 

396.  Geographical  Distribution  of  Auroras. — The  geographical 
distribution  of  auroras  depends  chiefly  upon  the  relative  intensity 
of  the  earth's  magnetism  in  different  latitudes.  According  to  ex- 
periments with  artificial  magnets,  the  electric  light  tends  to  form 
a  ring  around  the  pole,  and  at  some  distance  from  it.  The  elec- 
tric light  should  therefore  be  most  noticeable  in  the  neighbor- 


200  METEOKOLOGY. 

hood  of  the  earth's  magnetic  pole,  but  not  directly  over  it.  Au- 
roras are,  accordingly,  most  abundant  along  a  certain  zone  which 
follows  nearly  a  magnetic  parallel,  being  every  where  nearly  at 
right  angles  to  the  magnetic  meridian  of  the  place. 

397.  Why  Auroras  do  not  occur  within  the  Tropics. — The  elec- 
tricity of  the  tropical  regions  has  great  intensity,  and  moves  with 
explosive  violence  in  thunder -showers,  while  the  magnetic  in- 
tensity in  those  regions  is  very  feeble,  and  is  insufficient  to  con- 
trol the  movements  of  the  electricity.     In  the  higher  latitudes 
thunder-showers  become  infrequent,  the  electricity  of  the  atmos- 
phere passes  to  the  earth  in  a  slow  and  quiet  manner,  and  these 
discharges  are  controlled  by  the  magnetism  of  the  earth. 

398.  Cause  of  the  simultaneous  Displays  in  lot/i  Hemispheres. — 
We  can  not  explain  the  great  auroral  displays  in  the  northern 
hemisphere  by  supposing  that  the  electricity  of  the  atmosphere 
is  temporarily  diverted  from  one  hemisphere  to  the  other,  for  the 
mean  range  of  the  magnetic  needle  exhibits  its  maxima  simul- 
taneously in  both  hemispheres;  neither  can  we  suppose  that  the 
absolute  amount  of  electricity  for  the  entire  globe,  as  developed 
by  evaporation  from  the  water  of  the  ocean,  should  undergo  great 
periodical  variations,  for  the  mean  temperature  of  the  earth's  sur- 
face does  not  change  sensibly  from  one  year  to  another.     We 
seem,  therefore,  compelled  to  ascribe  these  great  auroral  displays 
in  no  small  degree  to  the  direct  action  of  the  sun  through  the 
agency  perhaps  of  its  magnetism,  or  of  the  electric  currents  cir- 
culating around  it.     Such  an  effect  should  take  place  simultane- 
ously in  both  hemispheres. 

399.  Possible  System  of  Electrical  Circulation. — Hence  it  appears 
probable  that  great  auroral  displays  are  not  exclusively  atmos- 
pheric phenomena,  but  are  to  some  extent  the  result  of  the  in- 
fluence of  extra  terrestrial  forces.     But,  if  these  extraordinary 
electrical  currents  are  mainly  determined  by  celestial  forces,  then, 
since  the  earth  exhibits  many  of  the  properties  of  a  permanent 
magnet,  the  two  magnetic  poles  of  the  earth  ought  to  exert  op- 
posite influences,  and  we  should  expect  that  the  currents  in  the 
neighborhood  of  the  two  poles  would  move  in  contrary  direc- 
tions.    Hence  we  naturally  infer  a  system  of  circulation  similar 


OPTICAL   METEOROLOGY. 


201 


Pig.  78.  to  that  which  is  represented 

by  Fig.  78,  where  N  and  S 
are  supposed  to  represent  the 
north  and  south  magnetic 
poles  of  the  earth,  n  and  s 
the  poles  of  an  imaginary 
magnet  representing  the 
magnetism  of  the  earth.  The 
east  and  west  bands  repre« 
sent  auroral  arches  upon 
which  stand  auroral  stream- 
ers. The  dotted  lines  repre- 
sent magnetic  curves  passing 
from  auroral  streamers  in 
the  southern  hemisphere  to 
streamers  in  the  northern 
hemisphere,  showing  the 
path  pursued  by  the  currents 
of  electricity  in  passing  from 
one  hemisphere  to  the  other, 
above  the  atmosphere. 

This  hypothesis  agrees  sub- 
stantially with  that  stated  in 
Art.  386,  so  far  as  the  phe- 
nomena can  be  observed  in  the  northern  hemisphere,  but  they 
lead  to  different  results  in  the  southern  hemisphere.  We  have 
not  the  requisite  observations  from  the  southern  hemisphere  to 
enable  us  to  decide  between  these  two  hypotheses. 


CHAPTER  YIIL 

OPTICAL   METEOROLOGY. 

SECTION  I. 

MIRAGE. 

400.  Mirage  is  an  atmospheric  phenomenon  which  produces  an 
apparent  displacement  of  distant  objects,  sometimes  elevating  and 
sometimes  depressing  them ;  sometimes  leaving  the  image  erect, 
and  sometimes  inverting  it,  as  when  objects  are  seen  reflected 


202  METEOROLOGY. 

from  a  lake  of  tranquil  water.  It  is  frequently  observed  on 
sandy  plains  intensely  heated  by  the  sun,  especially  in  Egypt  and 
Arabia. 

Lower  Egypt  is  a  vast  sand  plain,  with  occasional  villages  situ- 
ated upon  small  eminences.  In  the  middle  of  the  day,  these  vil- 
lages, seen  from  a  distance,  appear  as  if  situated  in  the  midst  of  a 
lake,  in  which  are  seen  the  inverted  images  of  houses  and  trees. 
The  outline  of  these  images  is  a  little  indistinct,  often  exhibiting 
an  undulatory  motion,  as  if  reflected  from  agitated  water.  As 
the  spectator  approaches  the  boundary  of  the  apparent  lake,  the 
waters  seem  to  retire,  and  the  same  illusion  appears  around  the 
next  village.  Similar  phenomena  are  common  in  some  parts  of 
California,  and  are  occasionally  seen  in  all  parts  of  the  United 
States.  Fig.  79,  p.  203,  represents  the  mirage  as  seen  in  Abyssinia. 
Sometimes  at  sea,  when  a  ship  is  barely  visible 
in  the  distant  horizon,  we  perceive  above  the 
ship,  A,  Fig.  80,  its  inverted  image,  B,  and  perhaps 
above  that  again  a  second  erect  image,  C.  Some- 
times of  the  two  upper  images  only  the  invert- 
ed one  is  seen,  and  sometimes  only  the  erect 
one. 

All  these  phenomena  are  due  to  unusual  varia- 
tions in  the  refractive  power  of  the  air,  arising 
from  extreme  changes  of  temperature.  The  mi- 
rage is  chiefly  seen  over  a  large  sandy  plain,  or 
over  water. 

401.  Mirage  upon  a  Desert. — Imagine  a  sandy  plain  nearly  hor- 
izontal, and  intensely  heated  by  the  rays  of  the  sun.  The  stratum 
of  air  which  rests  upon  the  sand  becomes  heated  by  it;  this  heat 
is  partially  communicated  to  the  superincumbent  strata,  so  that 
the  density  of  the  air  increases  rapidly  as  we  rise  above  the  earth 
up  to  a  moderate  height. 

Let  AB,  Fig.  81,  represent  a  tree  which  may  be  viewed  from  C 
in  its  true  position  through  air  of  nearly  uniform  density ;  and 
suppose  the  air  beneath  it  to  consist  of  strata  of  variable  density, 
decreasing  from  A  to  the  surface  of  the  ground.  The  rays  of 
light,  AD,  BE,  which  proceed  from  the  top  and  bottom  of  the 
tree,  passing  successively  through  strata  of  less  density,  will  be 
deviated  more  and  more  from  a  vertical  direction,  until  at  last 


OPTICAL   METEOKOLOGY. 
Fig.  79. 


203 


they  meet  a  stratum  at  such  an  angle  that  they  are  unable  to 
enter  it,  and  they  are  totally  reflected  from  this  stratum  at  D  and 

Fig.  81. 


E.  After  reflection,  these  rays,  traversing  strata  more  and  more 
dense,  will  be  refracted  upward,  and  at  C  reach  the  eye  of  the  ob- 
server, who  perceives  the  tree  in  the  direction  of  the  last  refracted 
rays.  An  image,  A'B',  will  therefore  be  seen  below  the  real  ob- 
ject, and  it  will  appear  inverted,  because  the  rays  have  suffered 
reflection.  The  effect  is  similar  to  that  produced  by  the  reflec- 
tion of  a  tree  from  the  surface  of  a  tranquil  lake,  and  the  ob- 
server is  thus  led  to  imagine  himself  to  be  surrounded  entirely 
by  water. 

Since  the  difference  of  refraction  of  the  successive  strata  of  air 


204  METEOROLOGY. 

is  necessarily  small,  the  ray  AD  must  be  very  oblique ;  that  is, 
the  object  must  be  elevated  but  little  above  the  ground,  and  the 
observer  must  be  at  a  considerable  distance. 

402.  Experimental  Illustration. — The  mirage  may  be  imitated 
artificially  by  superposing  in  the  same  vessel  two  liquids  of  differ- 
ent densities,  such  as  water  and  alcohol,  or  water  and  sirup  of 
sugar,  or  simply  cold  and  warm  water.     These  liquids,  by  partial 
mixture,  produce  a  medium  whose  refractive  power  decreases 
from  the  alcohol  to  the  water,  so  that  by  looking  through  this 
mixture  at  an  object  held  behind  the  vessel,  an  inverted  image  of 
it  may  be  seen. 

When  a  sandy  plain  is  intensely  heated  by  the  sun,  and  the  air 
is  very  calm,  if  we  place  the  eye  near  the  ground  we  may  gen- 
erally see  the  inverted  image  of  grass  and  other  objects  at  a 
distance. 

403.  Mirage  at  Sea. — Mirage  is  produced  at  sea  when  the  at- 
mosphere is  perfectly  calm,  and  the  air  in  contact  with  the  water 
is  colder  and  consequently  denser  than  the  stratum  of  air  imme- 
diately above  it;  this  second  stratum  is  denser  than  the  one  next 
above  it,  and  so  on.     In  such  a  case,  an  inverted  image  of  a  dis- 
tant object,  as  a  ship,  may  be  seen  with  a  distinctness  almost  equal 
to  that  of  the  object  itself,  and  this  image  will  be  formed  above 
the  object. 

Let  AB,  Fig.  82,  represent  a  ship  near  the  horizon  seen  in 
Fig.  82.  its  true  position  by 

direct  rays  coming 
to  the  eye  at  E, 
through  strata  of 
air  of  nearly  uni- 
form density.  Sup- 
pose the  air  consists 
of  strata  of  variable 
density,  the  density 
diminishing  rapidly 
from  below  upward.  The  rays  of  light,  AD,  BC,  which  proceed 
from  the  top  and  bottom  of  the  ship,  passing  from  a  denser  to  a 
rarer  medium,  will  be  deviated  more  and  more  from  a  vertical, 
until  at  last  they  meet  a  stratum  so  obliquely  that  they  are  unable 


OPTICAL  METEOROLOGY.  205 

to  enter  it,  and  are  totally  reflected  from  this  stratum  at  D  and  C. 
These  rays,  in  passing  from  the  rarer  to  the  denser  medium,  are 
now  refracted  downward,  and  meet  the  eye  at  E,  which  perceives 
the  ship  in  the  direction  of  the  last  refracted  rays,  and  the  object 
appears  inverted  because  the  rays  have  suffered  reflection. 

Other  rays,  that  never  could  reach  the  eye  at  E  in  the  ordinary 
state  of  the  atmosphere,  may  likewise  be  bent  into  curves  which 
do  not  cross  before  reaching  the  eye.  In  this  case  an  erect  image 
of  the  ship  may  be  seen,  and  both  the  direct  and  inverted  images 
may  be  seen  simultaneously. 

404.  Lateral  Mirage. — In  mountainous  countries,  or  near  a  high 
coast,  it  may  happen  that  the  air  is  divided  by  a  nearly  vertical 
plane  into  two  portions,  one  of  which  is  heated  by  the  sun,  while 
the  other  is  in  the  shadow  of  a  hill  or  a  bank.     The  transition 
from  the  warm  to  the  cold  air  will  not  be  abrupt,  but  the  density 
of  the  vertical  sections  will  increase  gradually  from  the  warmer  to 

Fig.  ss.  the  colder  portion.     If  an  ob- 

server were  situated  at  B,  Fig. 
83,  near  this  bounding  plane, 
he  might  see  in  the  warmer 
part  a  symmetrical  image,  C'D', 
of  objects,  CD,  situated  in  the 
colder  part,  as  if  in  a  vertical 
mirror.  This  is  called  a  later- 
0  al  mirage.  It  is  less  frequent- 

ly seen  than  the  other  varieties,  and  its  duration  is  more  transient. 

405.  Displacements. — Under  certain  circumstances,  objects  near 
the  horizon  may  appear  displaced ;  sometimes  laterally,  as  in  the 
vicinity  of  mountains,  but  more  frequently  in  a  vertical  direction, 
in  which  case  they  appear  elevated  above  their  true  position. 
Sometimes  an  object  appears  double,  certain  rays  reaching  the 
eye  without  sensible  deviation,  while  others,  traversing  strata  of 
increasing  density,  describe  a  curve  line.     This  phenomenon  dif- 
fers from  the  true  mirage  in  this  respect,  that  the  image  is  not 
inverted,  showing  that  the  light  has  not  suffered  reflection. 


206  METEOROLOGY. 


SECTION   II. 

ABSORPTION  AND  REFLECTION   OF  LIGHT  BY  THE   ATMOSPHERE. 

406.  Absorption  of  Light. — The  atmosphere  is  never  perfectly 
transparent,  but  absorbs  a  portion  of  the  light  which  traverses  it. 
Hence  distant  objects,  as  the  summits  of  mountains,  generally  ap- 
pear dim,  as  if  enveloped  in  a  mist  or  a  bluish  smoke.     This  loss 
of  light  is  due  partly  to  the  presence  of  minute  particles  of  con- 
densed vapor,  and  also  small  particles  of  dust,  and  partly  to  the 
difference  of  density  of  the  strata  arising  either  from  a  difference 
of  pressure  or  a  difference  of  temperature.     In  passing  from  one 
stratum  to  another  of  a  different  density,  a  portion  of  light  is  re- 
flected, so  that  the  transmitted  portion  is  continually  diminished. 
After  a  rain,  when  by  a  general  mingling  of  the  strata  the  tem- 
perature of  the  air  has  been  rendered  nearly  uniform,  its  trans- 
parency is  greatly  increased. 

407.  Redness  of  the  Evening  SJcy. — The  redness  of  the  evening 
sky  is  due  principally  to  the  condensed  vapor  of  the  air,  a  portion 
of  which  begins  to  be  precipitated  as  the  temperature  of  the  day 
declines. 

If  we  transmit  the  sun's  light  through  a  glass  prism  at  different 
hours  of  the  day,  we  shall  find  that  the  spectrum  changes  with 
the  altitude  of  the  sun.  As  the  sun  approaches  the  horizon,  the 
violet  part  of  the  spectrum  contracts,  and  at  length  disappears  al- 
together, while  the  red  end  of  the  spectrum  remains  entire.  We 
hence  conclude  that  the  violet  rays,  which  are  the  most  refrangi- 
ble, have  the  least  power  of  penetrating  the  dense  atmosphere,  in- 
cluding the  dust  and  the  condensed  vapor  near  the  horizon,  and 
therefore,  when  the  sun  is  near  the  horizon,  his  light  exhibits  an 
excess  of  those  rays  bordering  upon  the  red  end  of  the  spectrum, 
and  this  color  is  communicated  not  only  to  the  evening  sky,  but 
also  to  the  clouds  which  float  in  the  atmosphere. 

From  the  same  cause,  the  sun,  just  before  setting,  sometimes  as- 
sumes a  deep  red  color,  as  if  seen  through  a  smoked  glass,  and 
this  redness  is  more  noticeable  in  the  setting  than  in  the  rising 
sun,  because  in  the  morning  the  condensed  particles  of  vapor  have 
descended  to  the  earth,  or  are  converted  again  into  invisible  va- 
por by  the  increasing  heat  of  the  morning. 


OPTICAL   METEOROLOGY.  207 

408.  Reflected  Light  of  the  Sky. — An  observer  at  night,  in  the 
neighborhood  of  a  large  city,  may  notice  a  decided  illumination 
of  the  heavens,  arising  from  the  light  of  the  city  reflected  from 
the  sky,  and  during  an  extensive  conflagration  this  illumination 
is  sometimes  very  brilliant.     The  atmosphere  therefore  reflects  a 
portion  of  the  light  which  falls  upon  it.     It  is  this  light  of  the 
sky  which  prevents  our  seeing  the  stars  in  the  daytime,  and  its 
brightness  is  but  little  inferior  to  that  of  the  moon,  for  during  the 
day  the  moon  appears  like  a  small  white  cloud.     It  is  chiefly 
from  this  source  that,  during  the  day,  apartments  which  are  not 
accessible  to  the  direct  rays  of  the  sun  derive  their  illumination. 

The  brightness  of  the  sky  is  variable.  It  depends  upon  the 
purity  of  the  air,  increasing  with  the  number  of  the  particles  of 
condensed  vapor  suspended  in  it.  It  depends  also  upon  the 
weight  of  the  air  above  the  observer,  being  less  on  the  summits 
of  mountains  than  at  the  level  of  the  sea. 

The  light  of  the  sky  is  greatest  in  the  vicinity  of  the  sun,  and 
diminishes  rapidly  as  we  recede  from  his  disc. 

409.  Blue  Color  of  the  Sky. — While  the  red  rays  of  the  sun  have 
a  greater  power  of  penetrating  a  dense  atmosphere,  the  blue  rays 
are  more  readily  reflected  by  it,  but  this  difference  is  not  sensible 
until  the  light  has  traversed  large  masses  of  air.    The  azure  color 
of  the  sky  is  therefore  due  to  the  light  reflected  by  the  air,  and 
the  purer  the  air  the  more  decided  is  this  azure  tint.     When 
mountains  covered  with  snow  are  illumined  by  a  rising  sun,  they 
appear  of  a  rosy  or  orange  tint  upon  the  eastern  side,  while  on 
the  western  side  they  exhibit  a  bluish  tint.     The  blue  color  of 
the  sky  is  therefore  due  to  the  reflection  of  light,  and  not  to  a 
peculiar  color  belonging  to  the  particles  of  air. 

410.  Cyanometer. — The  intensity  of  the  blue  color  of  the  sky 
exhibits  very  great  variety.     In  order  to  measure  it,  Saussure  in- 
vented an  instrument  which  he  called  the  cyanometer.      This 
instrument  has  27  colored  surfaces,  of  which  the  first  is  almost 
white,  and  the  last  is  of  the  deepest  cobalt  blue,  while  the  inter- 
mediate surfaces  present  every  gradation  between  white  and  blue, 
and  the  surfaces  are  numbered  from  1  to  27.     It  has  also  a  sec- 
ond series  of  colored  surfaces,  beginning  with  the  deepest  blue  of 
the  preceding  series,  and  ending  with  a  jet  black,  while  the  inter 


208  METEOROLOGY. 

mediate  surfaces  present  every  gradation  between  blue  and  black, 
and  these  surfaces  are  numbered  from  27  to  53. 

In  using  this  instrument,  the  observer  selects  that  particular 
tint  upon  the  scale  which  corresponds  nearest  to  the  color  of  the 
sky,  and  the  color  of  the  sky  is  denoted  by  the  number  attached 
to  that  tint.  Other  cyanometers  have  been  invented  depending 
upon  the  properties  of  polarized  light. 

The  blueness  of  the  sky  generally  increases  from  the  horizon 
to  the  zenith.  When  the  color  of  the  sky  near  the  zenith  is 
indicated  by  20  on  the  cyanometer,  it  will  generally  be  about  4 
near  the  horizon. 

The  blueness  of  the  sky  is  greatest  after  a  rain,  when  the  air  is 
most  pure,  and  it  diminishes  with  an  increase  of  the  particles  of 
condensed  vapor  suspended  in  the  air.  Hence  a  pale  sky  is  a 
sign  of  rain. 

The  blueness  of  the  sky  decreases  as  we  recede  from  the  equa- 
tor. At  Cumana,in  latitude  10°,  the  average  blueness  of  the  sky 
is  24,  while  in  Europe  it  is  only  14.  On  a  clear,  bright  day,  the 
average  blueness  of  the  sky  at  New  Haven  corresponds  to  about 
18  on  the  cyanometer. 

The  blueness  of  the  sky  increases  with  the  altitude,  and  at  an 
elevation  of  16,000  feet  the  heavens  become  almost  black.  On 
the  top  of  Mount  Blanc,  Saussure  found  the  color  of  the  sky  39, 
while  at  the  foot  of  the  mountain  the  color  of  the  sky  near  thq 
zenith  was  represented  by  18. 

411.  Twilight. — If  there  were  no  atmosphere,  night  would  com- 
mence as  soon  as  the  sun  descends  below  the  horizon,  and  the 
day  would  begin  with  equal  abruptness.  The  astronomical  limit 
of  twilight  is  generally  understood  to  be  the  instant  when  stars 
of  the  sixth  magnitude  begin  to  be  visible  in  the  zenith  at  even- 
ing, or  disappear  in  the  morning.  In  our  climate  the  evening 
twilight  generally  terminates  when  the  sun  is  17°  or  18°  below 
the  horizon.  The  morning  twilight  commences  at  a  somewhat 
less  depression,  since  the  vapor  of  the  atmosphere  condensed  dur- 
ing the  night  does  not  rise  to  so  great  a  height  in  the  morning  as 
at  evening.  These  limits,  however,  are  variable,  the  duration  of 
twilight  depending  upon  the  state  of  the  atmosphere.  When  the 
sky  is  of  a  pale  color,  indicating  the  presence  of  an  unusual  amount 
of  condensed  vapor,  twilight  is  of  longer  duration.  This  happens 


OPTICAL  METEOROLOGY.  209 

habitually  in  the  polar  regions.  On  the  contrary,  within  the  trop- 
ics, where  the  air  is  pure  and  dry,  twilight  sometimes  lasts  only 
fifteen  minutes. 

412.  Twilight  Curve. — A  little  before  sunset  the  western  sky 
grows  yellow,  and  in  the  east  we  observe  a  purple  tint  arising 
from  the  reflection  of  the  sun's  rays,  which  have  traversed  the 
atmosphere  horizontally,  and  which  communicate  their  color  to 
whatever  they  illumine.     After  the  sun  has  set,  we  perceive  near 
the  eastern  horizon  a  dark  blue  segment,  above  which  we  notice 
the  purple  tint  already  mentioned.    As  the  sun  declines  this  seg- 
ment rises  higher;  it  subsequently  reaches  the  zenith,  and  finally 
the  western  horizon,  when  the  twilight  entirely  ceases.    The  out- 
line of  this  segment  sometimes  appears  very  sharply  defined,  and 
is  called  the  twilight  curve.     This  segment  is  a  part  of  the  coni- 
cal shadow  of  the  earth,  which  intercepts  the  sun's  rays  from  a 
portion  of  the  atmosphere,  and  this  portion  reflects  only  that  dif- 
fuse light  which  comes  from  other  parts  of  the  sky. 

413.  Colors  of  the  Morning  Twilight. — When  the  sun  is  still  12° 
below  the  eastern  horizon,  the  horizon  generally  appears  border- 
ed with  a  red  or  orange  band,  above  which  the  twilight  curve 
rises  7°.     The  orange  zone  gradually  extends,  becomes  bordered 
with  yellow,  and  afterward  with  green,  while  the  twilight  curve 
ascends  toward  the  zenith.     When  the  sun  is  only  2°  below  the 
horizon,  the  eastern  horizon  becomes  yellow,  the  green  zone  be- 
comes more  decided,  and  extends  from  3°  to  18°,  the  twilight 
curve  extends  to  within  3°  of  the  western  horizon,  and  is  border- 
ed with  a  purple  zone  about  12°  in  breadth.     As  the  sun  rises, 
the  western  horizon  appears  bordered  with  a  rosy  band,  sur- 
mounted by  yellow.     The  red  in  the  east  disappears,  and  is  suc- 
ceeded by  yellow,  surmounted  by  green,  which  continues  after 
*he  yellow  has  disappeared,  when  the  son  is  2°  or  4°  above  the 
horizon. 

The  red  and  yellow  zones  are  ascribed  to  the  absorption  pro- 
duced by  the  different  thicknesses  of  air  traversed  by  the  rays  of 
light.  The  green  color  results  from  a  combination  of  the  yellow 
rays  with  the  blue  rays  of  diffuse  light  reflected  from  the  parti- 
cles of  the  atmosphere,  green  being  produced  by  a  mixture  of 
yellow  and  blue. 

O 


210  METEOROLOGY. 

414.  Height  of  the  Atmosphere  deduced  from  Twilight. — Attempts 
have  been  made  to  compute  the  height  of  the  atmosphere  from 
the  position  of  the  twilight  curve  at  a  given  instant  after  sunset; 
but  the  results  thus  obtained  are  not  uniform,  being  greatest  when 
the  sun  is  lowest  below  the  horizon.    From  such  computations  it 
has  been  inferred  that  the  height  of  the  atmosphere  can  not  ex- 
ceed 36  miles;  but  this  can  only  be  regarded  as  the  height  of 
that  portion  of  the  atmosphere  which  has  a  density  sufficient  to 
reflect  an  appreciable  amount  of  light.     Other  phenomena  indi- 
cate that  an  extremely  rare  atmosphere  extends  to  a  much  great- 
er height. 

415.  Prognostics  derived  from  Twilight. — Since  the  colors  ano! 
duration   of  twilight,  especially   at   evening,  depend  upon    the 
amount  of  condensed  vapor  which  the  atmosphere  contains,  these 
appearances  should  afford  some  indication  of  the  weather  which 
may  be  expected  to  succeed.    The  following  are  some  of  the  rules 
which  are  relied  upon  by  seamen.     When,  after  sunset,  the  west- 
ern sky  is  of  a  whitish  yellow,  and  this  tint  extends  to  a  great 
height,  it  is  probable  that  it  will  rain  during  the  night  or  the 
next  day.     Gaudy  or  unusual  hues,  with  hard,  definitely  outlined 
clouds,  foretell  rain  and  probably  wind.    If  the  sun,  before  setting, 
appears  diffuse  and  of  a  brilliant  white,  it  foretells  a  storm.     If 
it  sets  in  a  sky  slightly  purple,  the  atmosphere  near  the  zenith 
being  of  a  bright  blue,  we  may  rely  upon  fine  weather. 

A  red  sky  in  the  morning  presages  bad  weather,  or  much  wind 
if  not  rain  ;  but  if  the  sky  presents  simply  a  rosy  or  grayish  tint, 
we  may  expect  fair  weather. 

SECTION  III. 

THE   RAINBOW. 

416.  The  rainbow  consists  of  a  series  of  circular  bands  colored 
with  the  tints  of  the  solar  spectrum  from  red  to  violet,  and  is  sit- 
uated in  that  part  of  the  sky  which  is  opposite  to  the  sun.     It  is 
caused  by  the  refraction  and  reflection  of  the  sun's  light  from 
drops  of  rain  whose  form  is  sensibly  spherical. 

It  is  proved  in  Natural  Philosophy  (Olmsted,  p.  419)  that,  if 
•i  represents  the  angle  of  incidence  of  a  ray  of  light, 
r         "         »         "        refraction         "        " 


OPTICAL   METEOROLOGY.  211 

D  represents  the  angle  of  deviation  of  a  ray  of  light, 
n  the  index  of  refraction  for  water; 

then,  for  the  maximum  deviation  after  one  reflection,  we  have 

I'tf—i      .      . 

cos.  i=\/ — - — ;  sin.  i  —  n  sin.  r;  D=4r— 2t. 
*       o 

If  we  assume  the  index  of  refraction  for  the  red  rays  to  be 
1.3309,  and  for  the  violet  rays  1.3442,  we  shall  find 

for  the  red     rays,  i=59°  32' ;  D=42°  24' ; 
"      violet  rays,  i==58°  46' ;  D=40°  28'. 
For  the  minimum  deviation  after  two  reflections  we  have 


/nz | 

cos.  i—  \J ;  sin.  f=n  sin.  r ;  D=«-+2t— 6r. 

V      8 

Whence,  by  computation,  we  find 

for  the  red      rays,  i-  71°  55' ;  D=50°  20' ; 
"      violet  rays,  i=  71°  29';  D=53°  46'. 

The  exterior  radius  of  the  primary  bow  should  therefore  be 
42°  24',  increased  by  half  the  diameter  of  the  sun;  and  its  breadth 
should  be  1°  56',  increased  by  the  apparent  diameter  of  the  sun, 
which  is  about  30',  making  2°  26'.  The  mean  of  numerous  care- 
ful measurements  gives  41°  33'  for  the  radius  of  the  middle  part 
of  the  primary  bow. 

The  interior  radius  of  the  secondary  bow  should  be  50°  20',  di- 
minished by  half  the  diameter  of  the  sun,  and  its  breadth  should 
be  3°  26' + 30',  or  3°  56'. 

417.  Necessary  Conditions  of  Visibility.  —  If  the  altitude  of  the 
sun  be  greater  than  the  radius  of  the  bow,  then  no  rainbow  can 
be  seen.     For  this  reason,  during  more  than  six  months  of  the 
year  at  New  Haven,  the  primary  bow  can  never  be  seen  at  noon, 
and  near  the  summer  solstice  the  primary  bow  can  not  be  seen 
for  more  than  six  hours  near  the  middle  of  the  day. 

If  the  observer  be  sufficiently  elevated  above  the  earth,  as  in  a 
balloon,  he  may  see  the  rainbow  as  a  complete  circle,  but  on  the 
surface  of  the  earth  we  only  see  a  semicircle  when  the  sun  is  in 
the  horizon. 

Lunar  rainbows  are  occasionally  seen,  but  the  colors  are  faint, 
and  generally  only  a  white  or  yellowish  arc  is  distinguishable. 

418.  Supernumerary  Bows. — The  Newtonian  theory  of  the  rain- 


212  METEOROLOGY. 

bow  is  incomplete,  inasmuch  as  it  only  considers  those  rays  which 
experience  the  maximum  or  minimum  deviation,  and  entirely 
neglects  those  rays  which  pass  a  little  beyond  these  limits.  The 
effect  of  these  other  rays  is  to  extend  the  breadth  of  the  primary 
bow  upon  the  inside,  and  also  to  produce  secondary  bands  which 
the  Newtonian  theory  does  not  explain.  When  the  rainbow  is 
brilliant,  we  often  perceive  faint  bauds  alternately  red  and  green 
within  the  violet  of  the  primary  bow,  or  perhaps  superposed  upon 
the  violet,  which  then  assumes  a  purplish  tint.  Near  the  violet 
bow  we  frequently  see  an  arch  of  rose-red,  succeeded  by  one  of 
yellowish-green ;  then  perhaps  a  second  arch  of  rose-red  and  a 
second  of  yellowish -green.  Two  supernumerary  bows  are  not 
very  uncommon ;  three  have  repeatedly  been  seen,  and  occasion- 
ally even  four. 

These  supernumerary  bows  are  due  to  the  interference  of  rays 
which  traverse  a  drop  in  a  direction  differing  but  little  from  that 
of  maximum  deviation.  To  every  angle  of  deviation  a  little  less 
than  the  maximum,  there  correspond  two  rays,  one  whose  angle 
of  incidence  is  a  little  greater,  and  the  other  whose  angle  is  a  lit- 
tle less  than  that  which  gives  the  maximum  deviation.  These 
rays,  having  pursued  routes  slightly  unequal,  interfere  and  pro- 
duce alternations  of  light  and  darkness,  or  alternately  bright  and 
dark  bands.  The  bands  resulting  from  these  interferences  for 
each  of  the  colors  of  the  spectrum,  being  superposed  upon  the 
sky,  produce  bands  analogous  to  the  colored  rings  of  thin  plates. 

419.  Theory  explained  from  a  Diagram.  —  A  ray  of  light,  SA, 
once  reflected  from  the  inner  surface  of  a  drop  of  rain  at  B,  ex- 
periences its  greatest  deviation,  viz., 
41°,  when  the  angle  of  incidence,  FE  A, 
is  59°.  Suppose  a  ray  of  light,  S'A', 
falls  upon  the  drop  at  an  angle  great- 
er than  59°,  it  will  experience  a  devia- 
tion less  than  41°.  So  also  a  ray,  S" 
A",  which  falls  upon  the  drop  at  an 
angle  less  than  59°,  will  experience  a 
deviation  less  than  41°.  That  is,  there 
are  always  two  rays  which  experience 
an  equal  deviation  (for  example,  40°), 
and  therefore  emerge  parallel,  one  of  them  making  with  the 


OPTICAL   METEOROLOGY.  213 

drop  an  angle  greater  than  59°,  and  the  other  a  less  angle.  The 
paths  of  these  two  rays  within  the  drop  are  slightly  unequal, 
and  there  are  two  rays  the  difference  of  whose  paths  within  the 
drop  is  equal  to  half  the  breadth  of  a  wave  of  light.  These 
waves,  being  in  opposite  phases,  will  interfere  with  each  other, 
and  produce  darkness.  There  are  two  other  rays  the  difference 
of  whose  paths  within  the  drop  is  equal  to  the  breadth  of  a  wave 
of  light.  These  waves,  being  in  the  same  phase,  will  conspire  to 
produce  a  double  illumination.  There  are  two  other  rays  the 
difference  of  whose  paths  is  equal  to  one  and  a  half  undulations, 
and  which  consequently  interfere  with  each  other. 

Thus  we  have  rays  the  difference  of  whose  paths  is  equal  to  1, 
2,  3, 4,  etc.,  undulations,  and  which  therefore  conspire ;  and  there 
are  other  rays  the  difference  of  whose  paths  is  equal  to  •£•,  H,  2^, 
3-j,  etc.,  undulations,  and  which  therefore  interfere. 

420.  Consequence  of  these  Interferences.  —  If,  then,  the  sun  fur- 
nished red  light  only,  we  should  see  opposite  to  the  sun,  when 
drops  of  rain  are  falling,  circular  arcs,  alternately  red  and  black. 
If  the  sun's  light  were  entirely  violet,  we  should  see  circular  arcs 
alternately  violet  and  black,  but  the  diameter  of  the  violet  arcs 
would  be  less  than  that  of  the  red  arcs.     The  other  colors  of  the 
spectrum  would  produce  arcs  of  intermediate  dimensions.     Now, 
since  the  sun's  light  contains  all  the  colors  of  the  spectrum,  all 
these  colored  arcs  are  in  fact  formed  simultaneously  and  super- 
posed, and,  being  of  unequal  diameters,  the  colors  are  partially 
blended.     But  near  the  usual  primary  bow  two  or  three  of  these 
narrow  bands  of  prismatic  colors  are  often  sufficiently  distinct  to 
be  visible.     In  consequence  of  this  reflection  of  light  from  the 
drops  of  rain,  it  results  that  the  sky  within  the  primary  bow  is 
brighter  than  that  without  it. 

421.  Size  of  tie  Drops  of  Rain. — The  smaller  the  drops  of  rain, 
the  broader  will  be  these  colored  bands.     In  order  that  the  su- 
pernumerary bows  may  be  formed  beyond  the  first  violet  bow, 
the  drops  must  be  extremely  minute.     It  is  found  by  computa- 
tion that  if  the  drops  be  ^V  inch  in  diameter,  a  second  red  band 
will  be  formed  2°  from  the  outer  red  of  the  primary  bow,  and  it  is 
near  this  point  that  the  first  supernumerary  bow  is  usually  seen. 

If  we  consider  the  interval  between  the  first  and  second  maxi- 


214  METEOROLOGY. 

mum  unity,  the  breadths  of  the  succeeding  intervals  for  the  same 
color  will  be  expressed  by  the  numbers, 

second  interval,  0.587 ;  fourth  interval,  0.440 ; 

third  interval,    0.493 ;  fifth  interval,     0.404. 

Supernumerary  bows  are  sometimes  seen  on  the  outside  of  the 
secondary  rainbow,  and  they  are  to  be  explained  in  a  similar 
manner. 

422.  Fog-low  explained. — If  the  drops  be  less  than  TV  inch  in 
diameter,  the  primary  bow  will  be  wider  than  two  degrees,  the 
breadth  of  the  bow  depending  simply  upon  the  size  of  the  drops. 
But  as  the  breadth  of  the  bow  increases,  the  colors  are  spread 
over  a  greater  surface,  and  consequently  they  are  less  vivid  and 
distinct.    When  the  diameter  of  the  drops  is  -o-jj-th  inch,  which  is 
the  average  diameter  of  particles  of  fog,  the  bow  becomes  a  very 
faint  arch  4°  or  5°  in  breadth,  with  only  a  slightly  rosy  tint  upon 
the  outside.     Such  is  the  bow  actually  observed  when  the  sun 
shines  upon  a  dense  fog. 

The  undulatory  theory  of  light,  therefore,  explains  not  only 
the  supernumerary  bows,  but  the  variable  breadth  of  the  prima« 
ry  bow. 

SECTION  IV. 

CORONA. 

423.  The  sun  and  moon,  when  partially  covered  by  light,  flee- 
cy clouds,  are  often  seen  encircled  by  one  or  more  colored  rings, 
which  are  called  coronce.     This  phenomenon  is  most  frequently 
noticed  about  the  moon,  since  we  are  too  much  dazzled  by  the 
light  of  the  sun  to  distinguish  faint  colors  surrounding  his  disc. 
In  order  to  examine  coronae  about  the  sun,  it  is  best  to  view  them 
by  reflection  from  a  blackened  mirror,  by  which  means  the  bril- 
liancy of  the  sun's  light  is  very  much  reduced. 

424.  Order  of  the  Colors. — When  a  corona  is  complete,  we  may 
observe  several  concentric  colored  circles.     The  one  next  to  the 
sun  is  blue,  the  second  is  nearly  white,  and  the  third  is  red. 
These  form  the  first  series  of  rings.     In  the  second  series  the  or- 
der of  the  colors  is  purple,  blue,  green,  pale  yellow,  and  red.     In 
the  third  series  the  colors  are  pale  blue  and  pale  red.     These 
rings  are  partially  represented  in  Fig  88 


OPTICAL   METEOROLOGY.  215 

The  diameter  of  these  rings  is  not  always  the  same.  The  di- 
ameter of  the  first  red  ring  varies  from  3°  to  6°,  and  that  of  the 
second  red  ring  from  5°  to  10°. 

425.  Cause  ofdoronce. — Coronas  are  produced  by  the  diffraction 
of  the  rays  of  light  in  their  passage  through,  the  small  intervals 
between  the  particles  of  condensed  vapor  in  a  cloud.    If  we  look 
at  the  moon  through  a  very  small  aperture  (as  a  pinhole  in  a 
plate  of  sheet-lead)  we  shall  see  the  hole  surrounded  by  colored 
rings,  whose  tints  are  the  same  as  those  observed  in  coronas.    The 
light  of  the  moon,  passing  through  the  small  interstices  between 
the  particles  of  a  cloud,  is  diffracted  in  a  similar  manner.     The 
particles  of  a  cloud  must  not  be  too  numerous,  otherwise  no  rays 
can  pass  between  them;  and  the  smaller  the  intervals  between  the 
particles,  the  greater  will  be  the  diameter  of  the  rings. 

426.  Corontz  produced  artificially. — If  we  sprinkle  upon  a  pane 
of  glass  a  little  lycopocliurn,  or  any  very  fine  dust  of  nearly  uni- 
form fineness,  and  look  at  the  moon  through  this  glass,  we  shall 
see  it  surrounded  by  rings  of  the  prismatic  colors,  precisely  like 
those  formed  by  a  cloud. 

If,  on  a  cold  winter  evening,  we  breathe  upon  a  pane  of  glass, 
the  breath  will  condense  in  small  globules  and  freeze;  and  if  we 
look  at  the  moon,  or  even  at  a  street  lamp,  through  this  glass,  we 
shall  see  a  similar  system  of  colored  rings,  having  violet  on  the 
inside. 

427.  Glow  surrounding  the  Shadow  of  an  Observer. — When  the 
sun  is  near  the  horizon,  and  the  shadow  of  the  observer  falls  on 
grass  covered  with  dew,  one  may  often  observe  a  vivid  glow  sur- 
rounding the  shadow  of  his  head.     If  the  shadow  falls  upon  a 
cloud  or  a  fog,  the  head  will  appear  surrounded  by  a  luminous 
glory,  exhibiting  the  prismatic  colors.     The  order  of  the  colors  is 
the  same  as  in  coronae,  and  sometimes  four  and  even  five  series 
of  rings  have  been  observed. 

The  light  of  the  sun  is  reflected  to  the  eye  most  powerfully  by 
the  particles  of  fog  near  the  head ;  for  the  light  reflected  both 
from  the  anterior  and  posterior  face  of  such  particles  will  reach 
the  eye.  This  explains  the  glow  of  light  surrounding  the  shad- 
ow of  the  observer.  The  color  is  produced  by  the  diffraction  of 
the  light  thus  reflected,  precisely  as  in  the  case  of  a  corona. 


216 


METEOROLOGY. 


SECTION  V. 

HALOS  AND   PARHELIA. 

428.  Halos  are  circles  of  prismatic  colors  formed  around  the 
sun  or  moon.  They  are  of  larger  size  than  coronas,  and  present 
a  greater  variety  of  appearances.  The  following  is  an  enumera- 
tion of  those  which  are  most  frequently  seen. 

Halo  of  22°  Radius.  —  When  the  sky  is  hazy,  and  presents  a 
dull,  rnilky  appearance,  we  frequently  notice  around  the  sun  or 
moon  a  colored  circle,  A,  Fig.  85,  having  a  radius  of  22°,  the  sun 

Fig.  85. 


occupying  the  centre  of  the  circle.  The  inner  edge  of  the  circle 
is  colored  red,  and  is  tolerably  well  defined ;  the  outer  edge  is  of 
a  pale  blue,  and  is  not  sharply  defined.  Such  a  circle  is  never 
seen  when  the  sky  is  perfectly  clear.  The  sky  within  the  halo 
is  much  darker  than  it  is  for  a  distance  of  several  degrees  without 
the  halo. 

The  light  of  this  halo  is  always  polarized  in  the  direction  of  a 
tangent  to  the  circumference,  which  proves  that  its  light  has  suf- 
fered refraction  and  not  reflection. 

429.  Theory  of  this  Halo. — This  halo  is  formed  by  the  refraction 
of  the  light  of  the  sun  or  moon  through  crystals  of  ice  floating  in 
the  atmosphere.  Snow  consists  of  crystals  of  ice.  The  simplest 
form  of  an  ice-crystal  is  a  right  prism,  whose  section  is  a  regular 
hexagon,  and  terminated  by  two  bases  perpendicular  to  the  edges 
of  the  prism.  The  alternate  faces  of  such  a  prism  are  inclined  to 


OPTICAL   METEOROLOGY. 


217 


Fig.  86. 
G 


Fig.  8T. 


each  other  at  angles  of  60°,  so  that  we 
may  consider  the  hexagonal  prism  ABC 
DEF  as  a  triangular  one,  GHK,  with  an- 
gles of  60°. 

When  a  ray  of  light  passes  through  a 
prisrn,  it  is  deviated  toward  the  base  of 
the  prism ;  and  there  is  a  certain  position 
of  the  prism  in  which  the  deviation  is  the 
least  possible.  This  deviation  for  a  prism, 
of  ice  may  be  computed  in  the  following  manner: 

Let  i  represent  the  angle  of  incidence 
of  a  ray  of  light ;  r  the  angle  of  refrac- 
tion ;  m  the  index  of  refraction  ;  and  A 
the  refracting  angle  of  the  prism.  Then 

sin.  i=m  sin.  r. 

But  when  the  deviation  is  a  minimum, 
r=30°;  and  for  red  light  the  value  of  m  is  1.307.  Hence  i— 40° 
48-|-'.  And  the  deviation  of  a  ray  of  light  is 

2i— A,  which  equals  21°  37'. 

The  minimum  deviation  for  the  violet  rays,  for  which  the  val- 
ue of  m  is  1.317,  is  found,  in  like  manner,  to  be  22°  22'. 

430.  How  a  Circle  of  Light  is  formed. — If  we  conceive  a  beam 
of  light  to  be  admitted  through  a  small  aperture  into  a  dark  room, 
and  to  full  upon  a  large  number  of  ice  prisms  having  angles  of 
60°,  and  occupying  every  possible  position,  all  the  incident  rays 
will  be  deviated  from  their  first  direction,  but  in  no  case  will  the 
deviation  be  less  than  about  22°.  A  large  number  of  spectra 
will  be  cast  upon  the  opposite  wall,  but  opposite  to  the  aperture 
through  which  the  light  is  admitted  there  will  be  a  circle  of  22° 
radius  upon  which  no  spectrum  can  fall,  and  the  red  end  of  each 
spectrum  will  be  turned  toward  the  centre  of  the  circle.  If  the 
number  of  the  spectra  be  sufficiently  great,  they  will  together 
form  a  circle  of  22°  radius,  bordered  with  red  upon  the  inside; 
but  beyond  the  red  the  different  colors  of  the  spectrum  will  be 
so  superposed  as  to  produce  a  light  nearly  white. 

Whenever  halos  are  formed  about  the  sun,  the  air  is  filled  with 
fine  prismatic  crystals  of  ice,  and  these  crystals  occupy  every  pos- 
sible position  with  respect  to  the  sun's  light.  The  halo  of  22° 
radius  is  formed  by  the  light  of  the  sun  shining  through  these 


218  METEOROLOGY. 

crystals  of  ice.  If  the  sun's  light  furnished  only  red  lays,  we 
should  have  an  illuminated  surface  with  a  circular  opening  of 
21^°  radius,  of  which  the  inner  edge  would  be  quite  light.  If 
the  sun  furnished  only  violet  rays,  we  should  have  a  similar  vio- 
let surface,  with  a  circular  opening  of  22-j°  radius,  and  the  inter- 
mediate colors  would  furnish  circles  of  intermediate  dimensions.. 
Now,  since  the  sun's  rays  contain  all  the  colors  of  the  spectrum, 
these  different  circles  are  formed  simultaneously  and  superposed. 
The  red  projecting  on  the  inside  is  unmingled  with  any  other 
color,  and  is  therefore  pure ;  all  the  other  colors  are  more  or  less 
mingled,  but  in  unequal  proportions,  so  that  the  outer  portion  of 
the  halo  is  nearly  white. 

Such  a  halo  may  be  formed  in  midsummer,  because  at  a  mod- 
erate elevation  above  the  earth's  surface  the  condensed  vapor  of, 
the  air  is  frozen  even  in  the  hottest  weather.  The  circle  within 
the  halo  is  much  darker  than  the  space  without  it,  because  from 
no  part  of  this  circle  can  a  ray  of  the  sun  refracted  by  ice  prisms 
reach  the  eye  of  the  observer. 

The  mean  of  eighty-three  measurements  of  the  radius  of  the 
red  circle  of  this  halo  is  21°  36',  which  is  almost  identical  with 
the  radius  computed  from  theory. 

431.  Hah  0/4:6°  Radius. — Sometimes  we  notice  around  the  sun 
a  second  colored  circle,  H,  Fig.  85,  having  a  radius  of  46°.     The 
inner  edge  of  this  circle  is  also  red,  and  tolerably  well  defined, 
while  the  outer  edge  is  of  a  pale  blue  color,  and  is  poorly  defined. 

This  halo  is  formed  by  the  refraction  of  the  sun's  rays  through 
ice  prisms  having  an  angle  of  90°,  this  being  the  angle  which  each 
side  of  the  hexagonal  prism  forms  with  its  base.  The  minimum 
deviation  of  a  ray  of  red  light  through  a  prism  of  ice  having  such 
a  refracting  angle  is  found  by  computation  to  be  45°  6',  and  for  a 
ray  of  blue  light  46°  50'.  The  average  of  the  best  observations 
give  45°  46'  as  the  radius  of  the  brightest  part  of  this  halo,  i\  coin- 
cidence as  exact  as  can  be  expected  in  observations  of  this  nature. 

432.  Halos  produced  artificially. — The  production  of  halos  may 
be  experimentally  illustrated  by  crystallizing  some  salt  like  alum 
upon  a  glass  plate,  and  then  looking  through  the  plate  at  the  sun 
or  a  candle.     A  few  drops  of  a  saturated  solution  of  alum  spread 
over  a  plate  of  glass  will  soon  cover  it  with  a  layer  of  minute 


OPTICAL  METEOROLOGY. 


219 


crystals.  If  we  place  the  eye  close  behind  the  smooth  side  of  the 
^lass,  and  look  at  a  candle,  we  shall  see  the  candle  surrounded 
by  three  halos  of  different  dimensions.  Each  crystal  of  alum  is  a 
regular  octaedron,  with  the  six  angles  truncated,  forming  the  out- 
line of  a  cube.  It  has  therefore  faces  inclined  to  each  other  at 
angles  of  70°,  90°,  and  110°,  and  these  angles  occupy  every  possi- 
ble position  with  respect  to  the  glass  plate.  The  smallest  halo  is 
formed  by  the  refraction  of  the  rays  of  light  through  a  pair  of 
faces  inclined  to  each  other  at  an  angle  of  70°;  the  second  halo 
is  formed  by  a  pair  of  faces  inclined  to  each  other  90°;  and  the 
third  halo  by  faces  inclined  at  an  angle  of  110°. 

433.  Halo  o/90°  Radius.— A.  third  halo  of  about  90°  radius,  H', 
Fig.  88,  is  occasionally  seen  surrounding  the  sun.  Unlike  the 
other  two  halos,  this  halo  shows  scarcely  any  traces  of  the  pris- 
matic colors.  Only  three  observations  of  this  halo  are  on  record, 


220  METEOROLOGY. 

and  its  exact  dimensions  have  not  been  well  determined.  In 
two  of  the  observations  the  radius  was  estimated  at  90°,  and  in 
the  third  it  was  estimated  from  85°  to  90°. 

This  halo  has  been  ascribed  to  rays  which,  after  entering  one 
of  the  sides,  AB,  of  a  triangular  ice  prism, 
meet  the  face,  BC,  at  such  an  angle  that 
they  are  totally  reflected,  and  emerge 
through  the  face  AC.  The  angle  of  total 
reflection,  r,  is  determined  by  the  equation 

1 

sin.  r= — . 

F  m 

For  violet  rays  in  ice,  m  =  1.3l7 ',  whence  r=49°  24',  or  BFE 
==40°  36'.  Hence  FEL  =  10°  36'. 

Also  KED  =  m  sin.FEL  =  14°  1'. 

The  inclination  of  DE  to  GH  is  equal  to  120°- 2.  KED  =  91e 
58'. 

Such  a  reflection  from  an  indefinite  number  of  ice  prisms  would 
therefore  furnish  an  illumined  surface  with  a  circular  opening  of 
about  92°  radius,  and  having  a  tinge  of  violet  on  the  side  next  to 
the  sun.  The  radius  above  computed  is  somewhat  greater  than 
that  indicated  by  the  observations,  and  there  are  other  objections 
to  this  explanation,  so  that  this  hypothesis  is  quite  doubtful;  but 
no  satisfactory  explanation  of  this  halo  has  hitherto  been  pro- 
posed, and  the  observations  are  not  sufficiently  precise  to  enable 
us  to  choose  between  conflicting  hypotheses. 

434.  Parhelic  Circle. — When  a  halo  is  formed  around  the  sun 
we  often  notice  a  white  circle  passing  through  the  sun  and  par- 
allel to  the  horizon.  See  Fig.  88.  This  is  called  a  parhelic  cir- 
cle, and  is  produced  by  the  reflection  of  the  sun's  light  from  ice 
prisms  or  snow  crystals  whose  surfaces  have  a  vertical  position. 
When  the  air  is  tranquil,  the  flakes  of  snow  which  are  present  in 
the  atmosphere  descend  slowly  to  the  earth,  and  they  tend  to  as- 
sume that  position  in  which  they  experience  the  least  resistance 
from  the  air.  For  most  forms  of  snow-flakes,  this  position  will 
be  when  the  principal  faces  of  the  crystal  are  perpendicular  to 
the  horizon,  and  the  light  of  the  sun  may  reach  the  eye  reflected 
from  such  snow-flakes  as  are  situated  on  a  horizontal  circle  pass- 
ing through  the  sun.  This  circle  never  exhibits  prismatic  colors 
like  the  first-mentioned  halos. 


OPTICAL  METEOROLOGY.  221 

435.  Parhelia. — Near  those  points  where  halos  cut  the  parhelia 
circle  there  is  a  double  cause  of  light,  and  here  the  illumination 
is  sometimes  so  great  as  to  present  the  appearance  of  a  mock  sun, 
and  is  called  a  parhelion.  Parhelia  are  generally  red  on  the  side 
which  is  toward  the  sun,  and  they  sometimes  have  a  prolongation 
in  the  form  of  a  tail  several  degrees  in  length,  and  whose  direc- 
tion coincides  with  that  of  the  horizontal  circle. 

Parhelia  of  22°. — The  number  of  parhelia  is  very  variable. 
One  is  commonly  seen  near  each  of  the  points  where  the  parhelic 
circle  cuts  the  halo  of  22°  radius,  £>p,  Fig.  88,  but  the  distance  of 
this  parhelion  from  the  sun  increases  with  the  elevation  of  the 
sun  above  the  horizon.  When  the  atmosphere  is  calm,  the  prisms 
of  ice  which  are  present  in  the  air,  and  are  slowly  descending  to 
the  earth,  will  tend  to  assume  a  vertical  position ;  and  if  the  sun 
be  near  the  horizon,  the  brightness  of  this  halo  will  be  greatest  at 
each  extremity  of  a  horizontal  diameter.  As  the  sun  rises  above 
the  horizon,  the  rays  of  light  traverse  these  vertical  prisms  in  a 
direction  oblique  to  the  axis,  and  the  minimum  deviation  of  a 
ray  is  increased,  and  the  parhelion  recedes  from  the  circumfer- 
ence of  the  halo.  For  an  elevation  of  20°  this  deviation  amounts 
to  a  degree  and  a  quarter ;  at  an  elevation  of  40°,  it  amounts  to 
more  than  five  degrees ;  and  at  an  elevation  of  about  50°,  this 
parhelion  entirely  disappears  on  account  of  the  oblique  angle  at 
which  the  rays  meet  the  ice  prisms. 

Parhelia  of  46°. — A  parhelion  is  sometimes  seen  at  each  of  the 
points  PP,  Fig.  88,  where  the  parhelic  circle  cuts  the  halo  of  46° 
radius.  These  parhelia  have  never  been  seen  to  depart  much  from 
the  circumference  of  the  halo;  but  since  the  breadth  of  the  halo 
is  l-j0,  and  that  of  the  parhelion  is  still  greater,  it  is  not  certain 
that  the  coincidence  is  exact.  These  parhelia  can  not  be  ascribed 
to  ice  prisms  with  angles  of  90°,  the  edges  of  these  angles  being 
vertical,  for  such  a  position  of  the  base  of  an  hexagonal  prism 
would  be  unstable.  Moreover,  upon  such  an  hypothesis,  as  the 
sun  rises  above  the  horizon,  the  parhelion  ought  to  recede  rapidly 
from  the  halo  of  46°,  which  is  contrary  to  observation. 

These  parhelia  are  probably  produced  by  rays  which  have  ex- 
perienced the  minimum  deviation  in  the  same  direction  in  two 
vertical  hexagonal  prisms,  in  which  case  the  total  deviation  of 
the  rays  would  be  double  of  that  produced  by  a  single  prism. 
Upon  this  hypothesis  the  parhelia  should  not  exactly  coincide 


222  METEOROLOGY. 

with  the  halo  of  46°,  but  forjelevations  not  exceeding  30°  the  dif- 
ference might  easily  escape  observation.  The  observations  are 
not  sufficiently  precise  to  decide  whether  this  explanation  is  ad- 
missible or  not. 

Parhelia  o/"120°. — Two  other  parhelia  are  sometimes  seen  on 
the  parhelic  circle,  about  120°  distant  from  the  sun.  These  may 
be  caused  by  two  reflections  of  the  rays  of  the 
sun  from  the  vertical  faces  of  snow  crystals, 
whose  form  is  such  as  is  represented  by  Fig. 
90.  The  ray  GH,  after  two  reflections  at  H 
and  K,  takes  the  direction  KL,  experiencing 
a  total  deviation  of  120°.  The  image  formed 
by  this  reflection  is  white,  and  its  size  about  equal  to  that  of  the 
sun's  disc. 

Parhelia  have  also  been  observed  at  distances  of  50°  and  98° 
from  the  sun,  which  may  result  from  the  reflection  of  the  sun's 
rays  from  the  faces  of  snow  en-  stals  of  more  complicated  forms. 

Sometimes  a  parhelion  is  seen  on  the  parhelic  circle  at  A,  Fig. 
88,  directly  opposite  to  the  sun.  This  is  more  properly  called  an 
antlielion. 

Phenomena  similar  to  parhelia  are  produced  by  the  light  of  the 
moon,  in  which  case  these  bright  spots  are  called  paraselene. 

436.  Contact  Arches.  —  Arcs  of  colored  circles  with  variable 
curvatures  are  sometimes  seen  touching  the  halos  of  22°  and  46° 
at  their  highest  and  lowest  points,  a,  6,  Fig.  88.  These  are  due  to 
the  refraction  of  the  sun's  light  through  ice  prisms,  some  of  them 
having  their  axes  perpendicular  to  the  sun's  rays,  and  others  in- 
clined at  various  angles,  but  all  in  a  horizontal  position.  The 
sun's  light,  refracted  by  such  prisms  as  have  their  axes  not  only 
horizontal,  but  perpendicular  to  the  solar  rays,  will  produce  a 
bright  image  directly  over  or  under  the  sun.  But  the  sun's  light, 
passing  through  prisms  whose  axes  are  inclined  to  the  solar  rays, 
will  experience  a  greater  deviation,  and  also  a  deflection  from  a 
vertical  plane.  Thus,  if  we  look  at  a  long  straight  bar  through 
a  prism  whose  axis  is  parallel  to  the  bar,  the  straight  bar  appears 
curved,  the  deviation  being  greatest  in  the  case  of  those  rays 
which  are  oblique  to  the  axis  of  the  prism. 


OPTICAL   METEOROLOGY.  223 

437.  Variable  form  of  Contact  Arches. — The  form  of  these  con- 
tact arches  depends  upon  the  height  of  the  sun  above  the  hori- 
zon.    When  the  sun  is  near  the  horizon,  we  sometimes  see  two 
brushes  of  light,  like  horns,  rising  from  that  point  in  the  halo  of 
22°  which  is  directly  over  the  sun.     As  the  sun  rises  higher, 
these  two  horns  diverge  from  each  other,  and  when  the  sun  has 
an  altitude  of  12°,  they  approach  in  form  to  an  arc  of  a  circle, 
with  its  convexity  toward  the  sun.     When  the  sun  reaches  an 
altitude  of  30°,  these  arcs  become  concave  toward  the  sun  ;  they 
bend  downward,  and  partially  envelop  the  halo. 

When  the  sun  has  an  altitude  of  25°,  a  contact  arch  is  some- 
times seen  at  the  point  of  the  halo  directly  beneath  the  sun.  At 
first  it  appears  like  an  arc  of  a  circle,  with  its  convexity  turned 
toward  the  sun.  As  the  sun  rises  higher,  the  curvature  of  this 
arc  diminishes,  and  at  an  altitude  of  32°  the  arc  becomes  concave 
toward  the  sun.  At  the  height  of  45°  the  curvature  of  the  lower 
contact  arch  is  nearly  the  same  as  that  of  the  upper  arch,  and 
both  together  form  an  elliptical  figure,  sur- 
rounding the  halo  of  22°,  as  shown  in  Fig.  91. 
When  this  ellipse  is  greatest,  the  length  of 
its  horizontal  axis  is  about  64°.  As  the  sun 
rises  still  higher,  the  major  axis  of  this  el- 
lipse contracts,  and  when  the  sun's  altitude 
is  60°,  the  horizontal  axis  of  the  ellipse  is  re- 
duced to  50°.  At  the  altitude  of  70°,  the  ellipse  differs  so  little 
from  the  halo  itself  as  to  be  scarcely  distinguishable  from  it. 

All  these  arcs  are  due  to  the  sun's  light,  refracted  by  ice  prisms 
having  their  axes  horizontal,  as  may  be  verified  experimentally 
by  passing  the  sun's  light  through  a  triangular  water  prism  held 
in  the  proper  position  with  respect  to  the  sun's  light. 

438.  Arcs  touching  the  Halo  o/"46°. — When  the  sun  has  an  alti- 
tude of  12°,  a  brilliant  arch  in  the  form  of  an  inverted  rainbow  is 
sometimes  seen  to  touch  the  halo  of  46°  at  its  highest  point,  6, 
Fig.  88.     As  the  sun  rises  higher  in  the  heavens  this  arc  be- 
comes more  curved,  and  it  disappears  when  the  sun  attains  an 
altitude  of  31°. 

When  the  sun  has  an  altitude  of  60°,  a  colored  arch  is  some- 
times seen  touching  the  halo  of  46°  at  its  lowest  point;  but  its 
li°rht  is  faint,  and  it  is  concave  toward  the  sun,  so  that  this  arc  is 


224  METEOROLOGY. 

easily  confounded  with  the  halo  itself.  As  the  sun  rises  higher 
in  the  heavens,  this  arch  approaches  still  nearer  to  coincidence 
with  the  halo,  and  it  disappears  entirely  when  the  altitude  of  the 
sun  is  78°. 

These  arcs  are  formed  by  the  refraction  of  the  sun's  light 
through  ice  prisms  having  angles  of  90°,  the  edges  of  these  an- 
gles being  situated  in  a  horizontal  plane ;  and  the  angles  will  have 
this  position  when  the  axis  of  the  hexagonal  prism  is  vertical. 
A  ray  of  the  sun  can  not  pass  through  so  large  a  refracting  an- 
gle except  when  the  sun  has  a  particular  altitude  above  the  hori- 
zon. It  is  for  this  reason  that  the  upper  contact  arch  is  never 
seen  except  when  the  sun's  altitude  is  between  12°  and  31°;  and 
the  lower  contact  arch  is  never  seen  except  when  the  sun's  alti- 
tude is  the  complement  of  the  preceding,  viz.,  from  59°  to  78°. 

439.  Intersecting  Arcs  opposite  to  the  Sun. — Sometimes  we  notice 
two  arcs  of  circles  nearly  white,  A,  Fig.  88,  intersecting  the  par- 
helic  circle  at  a  point  directly  opposite  to  the  sun,  and  inclined  to 
this  circle  at  angles  of  about  60°. 

They  are  probably  due  to  reflection  from  surfaces  oblique  to  the 
horizon;  perhaps  from  the  slender  spiculse  of  snow-flakes  having 
Fig. 92.  Fig. 93.          the  form  and  position  shown  in  Fig. 

92,  or  from  hexagonal  snow -plates 
whose  bases  are  covered  with  striae 
arising  from  the  superposition  of  oth- 
er hexagonal  plates  in  the  manner 


shown  in  Fig.  93.     Flakes  of  snow 
having  such  a  figure  have  been  repeatedly  observed. 

440.  Vertical  Columns  passing  through  the  Sun.  —  Sometimes, 
near  sunset,  we  notice  a  luminous  column,  perpendicular  to  the 
horizon,  rising  from  the  sun  to  a  height  of  10°  or  15°,  and  occa- 
sionally still  higher.  This  column  is  due  to  the  reflection  of  the 
sun's  light  from  the  under  faces  of  ice  crystals  which  are  nearly 
parallel  to  the  horizon.  Sometimes  a  little  before  sunset  a  sim- 
ilar column  of  light  is  seen  to  shoot  down  from  the  sun  toward 
the  horizon.  This  is  formed  in  a  similar  manner  by  rays  of  the 
sun  reflected  from  the  upper  faces  of  crystals  in  a  nearly  hori- 
zontal position.  Sometimes  columns  are  seen  simultaneously 
both  above  and  below  the  sun ;  and  if  the  halo  of  22°  is  seen  at 


SHOOTING-STARS,  METEORS,  AND  AEROLITES.  225 

rig.  94.  the  same  time,  this  column,  together  with  the 

parhelic  circle,  presents  the  appearance  of  a 
rectangular  cross  within  the  halo,  Fig.  94. 
These  luminous  columns  are  probably  formed 
only  when  the  air  is  very  tranquil,  and  the 
reflecting  surfaces   may   be   the    rectangular 
r'  terminations  of  spiculse  of  ice  which  are  slow- 
ly falling  to  the  earth,  with  their  axes  nearly 
in  a  vertical  position. 

When  we  remember  the  immense  variety  in  the  forms  of  snow- 
flakes,  a  few  of  which  are  represented  in  Fig.  52,  we  should  antici- 
pate a  very  great  variety  in  the  figures  which  might  be  produced 
from  the  refraction  or  reflection  by  them  of  the  sun's  light.  In 
addition  to  the  figures  which  have  been  described  in  this  section, 
many  others  have  occasionally  been  seen,  but  the  descriptions 
which  have  been  furnished  of  them  are  not,  in  general,  sufficiently 
precise  to  enable  us  to  decide  respecting  their  proper  explanation. 


CHAPTEE  IX. 

SHOOTING-STARS,  DETONATING  METEORS,  AND  AEROLITES. 

SECTION  I. 

SHOOTING-STABS. 

441.  /Shooting-stars  described. — The  term  shooting-star,  or  fall- 
ing-star, is  employed  to  designate  a  luminous  meteor  which  at 
night  is  frequently  seen  to  shoot  rapidly  across  the  sky,  and  pres- 
ently vanishes,  appearing  as  a  star  which  is  shot  away  from  its 
place  in  the  firmament  to  a  distant  region  of  the  heavens.    Shoot- 
ing-stars may  be  seen  on  every  clear  night,  and  at  times  follow 
each  other  so  rapidly  that  it  is  quite  impossible  to  count  them. 

442.  Number  seen  at  different  Hours.  —  Shooting -stars  are  not 
seen  with  equal  frequency  at  all  hours  of  the  night.     They  gen- 
erally  increase  in  frequency  from  the  evening  twilight  through- 
out the  night  until  the  morning  twilight ;  and  when  the  light  of 
day  does  not  interfere,  they  are  generally  most  numerous  about 
6  A.M.     The  following  table  shows  the  average  number  of  shoot- 

P 


226 


METEOROLOGY. 


6 

to 

7 

P 

.M, 

3.8 

From 

12 

to 

1 

7 

<( 

8 

u 

3.8 

it 

1 

u 

2 

8 

u 

9 

c( 

4.0 

a 

2 

(( 

8 

9 

u 

10 

u 

4.7 

t( 

3 

u 

4 

10 

" 

11 

u 

5.3 

n 

4 

« 

5 

11 

(( 

midnight, 

6.0 

u 

5 

u 

G 

ing-stars  which  may  be  seen  by  a  single  observer  at  each  hour  of 
a  clear  night,  in  the  absence  of  the  moon  : 

From    6  to    7  P.M.,        3.8         From  12  to    1  AM.,        7.2 

7.8 
8.7 

"         10.3 
"         11.2 
11.2 

Observations  show  that  the  whole  number  of  shooting -stars 
visible  at  one  place  must  be  at  least  six  times  the  number  which 
can  be  seen  by  one  observer.  Hence  the  average  number  of 
meteors  that  traverse  the  atmosphere,  and  that  are  large  enough 
to  be  visible  to  the  naked  eye,  if  the  sun,  moon,  and  clouds  would 
permit,  is  42  in  an  hour,  or  1000  daily. 

443.  Number  seen  in  the  different  Months.  —  Shooting-stars  are 
not  seen  with  equal  frequency  at  all  seasons  of  the  year.  The 
following  table  shows  the  average  hourly  number  which  may  be 
seen  by  a  single  observer  near  midnight,  during  each  month,  on 
clear  nights,  in  the  absence  of  the  moon : 


January  . 

.     5.1 

May   . 

February 

.     5.0 

June  . 

March 

.     4.8 

July  . 

April 

.    4.6 

August 

4.0 

4.9 
10.0 
12.8 


September  .  7.4 

October  .     .  7.7 

November  .  7.4 

December    ,  5.4 


"We  thus  see  that  many  more  shooting-stars  appear  from  July 
to  December  than  during  the  other  six  months  of  the  year;  and 
they  are  ordinarily  most  abundant  in  the  month  of  August. 

444.  Altitude  of  Shooting-stars. — If  two  observers,  at  a  suitable 
distance  from  each  other,  note  the  apparent  altitude  and  azimuth 
of  a  shooting-star  at  the  commencement  of  its  flight,  and  do  the 
same  also  for  its  termination,  we  have  the  data  for  computing  the 
absolute  height  of  beginning  and  end  above  the  surface  of  the 
earth.  The  earliest  observations  of  this  kind  were  made  in  1798 
by  Benzenberg  and  Brandes  in  Germany,  and  since  that  time  sim- 
ilar observations  have  been  made  in  many  parts  of  Europe,  and 
also  in  the  United  States.  It  is  found  that  when  the  base-line 
employed  is  only  three  or  four  miles  in  length,  a  shooting-star 


SHOOTING-STARS,  METEORS,  AND  AEROLITES.  227 

is  seen  in  nearly  the  same  direction  at  both  stations,  showing  that 
its  altitude  is  much  greater  than  the  length  of  that  base.  When 
the  base-line  is  30  or  40  miles,  the  average  change  of  position  of 
the  star  is  about  15°.  The  base-line  should  not  be  less  than  40 
or  50  miles  in  length,  and  one  of  75  or  100  miles  would  not  be  too 
great.  Observers  at  distances  of  over  150  miles  see  for  the  most 
part  different  shooting-stars. 

The  heights  of  over  500  meteor  paths  have  been  computed, 
and  it  is  thus  found  that  shooting-stars  begin  to  be  visible  at  ele- 
vations of  from  40  to  120  miles,  and  perhaps  sometimes  150  miles, 
or  an  average  height  of  74  English  statute  miles.  They  dis- 
appear at  elevations  of  from  30  to  80  miles,  and  perhaps  some- 
times 100  miles  or  more,  giving  an  average  height  at  disappear- 
ance of  52  English  statute  miles. 

445.  Length  of  Path  and  Velocity.  —  The  length  of  the  visible 
path  of  shooting-stars  varies  from  10  to  100  miles,  though  some- 
times they  are  even  300  and  400  miles  long;  the  average  length 
being  28  miles.     The  time  of  describing  the  visible  path  varies 
from  less  than  one  second  to  five  seconds,  and  in  some  rare  cases 
amounts  to  ten  seconds ;  but  their  average  duration  is  less  than 
one  second.     The  average  duration  of  meteors  whose  brightness 
exceeds  that  of  stars  of  the  first  magnitude  is  estimated  at  one 
and  a  half  seconds. 

Their  velocity  relative  to  the  earth's  surface  varies  from  10  to 
45  miles  per  second,  and  the  average  velocity  of  the  brighter  class 
of  shooting-stars  amounts  to  about  30  miles  per  second. 

446.  Direction   of  their  Motions.  —  Shooting-stars   are  seen  to 
move  in   all  directions  through  the  heavens.     Their  apparent 
paths  are,  however,  generally  inclined  downward,  though  some- 
times they  move  upward,  and  after  midnight  they  come  in  the 
greatest  numbers  from  that  quarter  of  the  heavens  toward  which 
the  earth  is  moving  in  its  annual  course  around  the  sun. 

447.  Magnitude  of  Shooting-stars. — The  magnitude  of  shooting- 
stars  is  very  variable.     Some  of  them  have  been  computed  to 
have  a  diameter  of  100  or  200  feet,  and  others  1000  up  to  5000 
or  6000  feet.     We  must,  however,  regard  this  as  the  diameter  of 
the  blaze  of  light  which  surrounds  the  meteor,  while  the  meteo" 


228  METEOROLOGY. 

itself,  before  it  takes  fire,  may  have  a  diameter  of  only  a  few  feet, 
or  perhaps  only  a  fraction  of  an  inch.  The  apparent  size  of  me- 
teors is  greatly  magnified  by  irradiation. 

448.  Visible  Train. — Occasionally  shooting-stars  appear  in  great 
splendor,  flashing  with  a  brightness  nearly  equal  to  that  of  the 
full  moon,  and  leaving  behind  them  a  train  of  dazzling  light, 
which  lasts  for  several  seconds,  and  even  for  whole  minutes. 
Their  color  is  usually  white,  with  a  reddish  tinge ;  but  occasion- 
ally they  exhibit  a  green  light,  and  sometimes  a  mixture  of  green 
and  blue,  or  purple.     Even  quite  faint  shooting-stars  sometimes 
leave  trains. 

The  path  of  shooting-stars  is  frequently  curved — sometimes  the 
path  consists  of  two  portions  inclined  to  each  other  at  a  consider- 
able angle — and  at  the  end  the  meteor  sometimes  bursts  like  a 
rocket  into  numerous  fragments.  In  such  cases  the  place  of  ex- 
plosion is  usually  indicated  by  a  smoky  cloud,  which  sometimes 
continues  visible  for  ten  minutes. 

449.  Are  Shooting -stars  accompanied  by  any  Sound? — Observers 
frequently  imagine  that  they  hear  a  whizzing  noise  accompany- 
ing the  passage  of  a  brilliant  meteor.     It  is  easily  proved  that 
such  impressions  are  an  illusion.     When  we  compute  the  path  of 
the  meteor,  from  which  the  sound  was  supposed  to  proceed,  we 
always  find  that  it  was  quite  distant  from  the  observer,  20,  or  50, 
and  perhaps  100  miles.     Now  sound  moves  with  a  velocity  of 
1120  feet  per  second,  or  50  miles  in  about  four  minutes.     If,  then, 
any  noise  was  caused  by  the  motion  of  the  meteor,  the  sound 
could  not  possibly  be  heard  until  considerable  time  after  the  me- 
teor disappeared,  viz.,  2,  5,  or  even  10  minutes,  according  to  its 
distance. 

450.  Cause  of  the  Light  of  Shooting-stars. — This  light  is  proba- 
bly due  to  the  high  temperature  resulting  from  the  resistance  of 
the  atmosphere  to  the  rapid  motion  of  the  meteor.     Since,  at  the 
ordinary  elevation  of  shooting-stars,  the  air  is  exceedingly  rare,  it 
might  be  supposed  that  the  resistance  would  not  develop  sufficient 
heat  to  give  meteors  their  brilliant  appearance.     The  researches 
of  philosophers  have  enabled  us  to  compute  the  quantity  of  heat 
that  may  be  develoned  by  the  stoppage  of  a  meteor  in  the  atmos- 


SHOOTING-STARS,  METEORS,  AND   AEROLITES.  229 

phere.  A  portion  of  the  living  force  of  the  body  is  expended  in 
setting  the  uir  in  motion,  and  a  portion  in  heating  the  meteor  and 
the  air.  This  living  force  and  the  consequent  heat  that  may  be 
developed  is  proportioned  to  the  mass  of  the  body  and  to  the 
square  of  its  velocity.  The  arresting  the  motion  of  a  meteor 
whose  velocity  is  thirty  miles  per  second,  and  whose  specific  heat 
is  0.12,  would,  if  the  whole  living  force  were  changed  into  heat, 
be  sufficient  to  raise  the  temperature  of  the  meteoric  body  more 
than  four  million  degrees  of  Fahrenheit's  scale.  If  even  the 
larger  part  of  this  force  was  expended  in  giving  motion  to  the 
air,  there  would  remain  enough  to  furnish  a  brilliant  light,  and  to 
melt  or  disintegrate  the  meteor. 

451.  Daily  number  of  Shooting-stars  for  the  whole  Globe.  —  The 
mean  distance  of  shooting-stars  from  the  observer  is  found  to  be 
about  105  miles.     The  average  height  above  the  earth  of  the 
middle  points  of  their  paths  is  63  miles.     Hence  the  mean  hori- 
zontal distance  of  the  paths  may  be  regarded  as  about  90  miles. 
It  is  reasonable  to  suppose  that  the  number  of  shooting-stars  ac- 
tually falling  within  a  circle  of  90  miles  radius  is  at  least  equal 
to  the  number  seen  at  one  place.     In  fact,  careful  computations 
show  that  it  is  about  one  fourth  greater.    The  area  of  this  circle  is 
25,447  miles,  while  the  entire  surface  of  the  globe  is  196,662,000 
square  miles.     The  ratio  of  these  numbers  is  7728,  whence  we 
may  safely  conclude  that  the  number  of  shooting-stars  over  the 
whole  earth  is  more  than  eight  thousand  times  the  number  visi- 
ble at  one  place. 

The  average  daily  number  of  shooting-stars  visible  to  the  naked 
eye  at  one  place  has  been  estimated  at  1000,  Art.  442.  Hence 
the  average  number  of  meteors  that  traverse  the  atmosphere 
daily,  and  that  are  large  enough  to  be  visible  to  the  naked  eye, 
if  the  sun,  moon,  and  clouds  would  permit,  must  be  more  than 
1000  x  8000,  or  more  than  eight  millions. 

452.  Number  of  telescopic  Shooting -stars. — The  observations  of 
Pape  and  Winnecke  indicate  that  the  number  of  meteors  visible 
through  the  comet-seeker  employed  by  the  latter  is  about  40  times 
the  number  visible  to  the  naked  eye.     A  further  increase  of  op- 
tical power  would  doubtless  reveal  a  still  larger  number  of  these 
small  bodies.     Hence  we  must  conclude  that  the  source  from 


230  METEOROLOGY. 

which  these  meteors  come  is  of  immense  extent,  otherwise  it 
would  long  since  have  been  exhausted. 

The  mass  of  these  bodies  is,  however,  so  small,  and  their  dis- 
tance from  each  other  so  great,  that  they  exert  no  appreciable  in- 
fluence upon  the  motion  of  the  planets.  It  is  computed  that  the: 
average  distance  from  each  other  of  shooting-stars,  such  as  under 
favorable  circumstances  would  be  visible  to  the  naked  eye,  is 
about  three  hundred  miles. 

453.  Meteoric  Orbits. — Having  determined  the  velocity  and  di- 
rection of  a  meteor's  path  with  reference  to  the  earth,  and  know- 
ing, also,  the  direction  and  velocity  of  the  earth's  motion  about 
the  sun,  we  can  compute  the  direction  and  velocity  of  the  motion 
with  reference  to  the  sun.     This  computation  has  been  made  for 
several  different  meteors,  and  has  shown  that  these  bodies,  before 
they  approached  the  earth,  were  revolving  about  the  sun  in  el- 
lipses of  considerable  eccentricity.     In  some  instances  the  veloci- 
ty has  been  found  to  be  so  great  as  to  indicate  that  the  path  dif- 
fered little  from  a  parabola. 

It  is  thus  demonstrated  that  ordinary  shooting-stars  are  small 
meteoric  bodies,  moving  through  space  in  paths  similar  to  the 
comets,  and  it  is  probable  that  they  do  not  differ  materially  from 
the  comets  except  in  their  dimensions,  and  perhaps,  also,  in  their 
density 

454.  Periodic  Meteors  of  November. — We  have  seen,  Art.  443, 
that  the  average  number  of  shooting-stars  for  the  different  months 
of  the  year  is  quite  unequal,  and  occasionally  the  display  of  me- 
teors is  very  extraordinary.     The  most  remarkable  exhibitions 
of  this  kind  during  the  last  two  centuries  have  occurred  in  No- 
vember.    On  the   morning  of  November  13,  1833,  throughout 
most  of  North  America,  shooting-stars  appeared  in  such  numbers 
that  it  was  found  impossible  to  count  them.     At  Boston  it  was 
estimated  that  the  meteors  fell  at  the  rate  of  575  per  minute. 
Most  of  these  meteors  moved  in  paths  which,  if  traced  backward, 
would  meet  in  a  single  point,  or  small  area,  situated  near  y  Le- 
onis. 

On  the  13th  of  November,  in  1832,  shooting-stars  appeared  in 
very  unusual  numbers,  and  there  was  a  moderate  display  on  the 
same  day  of  1834, 1835,  and  1836. 


SHOOTING-STARS,  METEORS,  AND  AEROLITES. 


231 


On  the  morning  of  November  12th,  1799,  an  extraordinary  fall 
of  shooting-stars  was  witnessed  in  South  America  by  Humboldt, 
and  it  was  also  seen  throughout  a  considerable  part  of  North 
America.  The  examination  of  old  historical  records  has  led  to 
the  discovery  of  at  least  ten  other  similar  appearances  at  about 
the  same  season  of  the  year.  These  occurred  in  the  years  902, 
931,  934,  1002,  1101,  1202,  1366,  1533,  1602,  and  1698. 


455.  Meteoric  Shower  of  November  14^A,  1866.  —  These  remarka- 
ble displays  having  occurred  at  intervals  of  33  or  34  years,  or 
some  multiple  of  that  period,  led  to  a  general  expectation  of  a 
brilliant  shower  in  1866.  At  New  Haven,  on  the  night  of  No- 
vember 13th-14th,  881  meteors  were  counted  in  five  hours,  which 
is  six  times  the  average  number  for  November;  but  a  far  more 
brilliant  display  was  witnessed  in  Europe.  On  the  morning  of 
November  14tb,  at  Greenwich,  from  midnight  to  1  o'clock,  there 
were  observed  2032  meteors;  from  1  to  2  o'clock,  4860  meteors; 
and  from  2  to  3,  832  meteors,  the  maximum  occurring  about  a 
quarter  past  one,  when  the  number  amounted  to  120  per  minute. 
The  curve  line,  Fig.  95,  shows  the  number  of  meteors  observed 


IO.PM 


each  minute  from  10  P.M.,  November  13th,  to  5  A.M.,  November 
14th,  the  number  visible  at  each  instant  being  indicated  by  the 
numerals  0  to  120  on  the  left  of  the  diagram.  Nearly  all  of  these 
meteors  proceeded  from  a  point  in  the  constellation  Leo;  thia 


232  METEOROLOGY. 

point  being  in  latitude  10°  N.,  and  its  longitude  being  about  90° 
less  than  that  of  the  sun. 

A  similar  display  was  noticed  throughout  Europe;  also  in  Asia 
as  far  eastward  as  Calcutta,  and  in  corresponding  longitudes  in 
the  southern  hemisphere.  Throughout  all  this  region  the  max- 
imum display  occurred  at  about  the  same  instant  of  absolute  time. 

456.  Meteoric  Shower  of  November  14th,  1867. — An  equally  re- 
markable display  of  meteors  occurred  in  the  United  States  on  the 
morning  of  November  14th,  1867.     Until  3  A.M.  the  number  of 
shooting-stars  was  not  remarkable,  but  from  that  hour  the  num- 
ber rapidly  increased,  and  at  New  Haven  attained  its  maximum 
about  4r|  A.M.,  after  which  the  number  declined,  and  before  six 
o'clock  had  ceased  to  be  specially  noticeable.     Near  the  time  of 
maximum  the  number  visible  to  a  single  person  was  43  per  min- 
ute, making  about  240  per  minute  for  the  entire  heavens,  and  this 
in  the  presence  of  a  full  moon,  which  probably  eclipsed  two  thirds 
of  those  which  would  otherwise  have  been  visible.     These  mete- 
ors almost  without  exception  moved  in  paths  which,  if  produced 
backward,  would  intersect,  not  all  precisely  in  a  single  point,  but 
within  a  small  area  situated  in  Leo.     This  area  was  of  an  oval 
form,  having  a  diameter  of  about  5°  in  longitude  and  1°  in  lati- 
tude.    Its  centre  was  in  longitude  143°,  and  latitude  10°  10'  N., 
and  most  of  the  meteors  appeared  to  diverge  pretty  accurately 
from  this  centre.     Many  of  them  left  trains  which  were  distinct- 
ly visible  for  several  seconds,  notwithstanding  the  light  of  the 
moon. 

457.  Procession  of  the  Node  along  the  Ecliptic. — The  day  of  the 
year  upon  which  the  great  displays  of  the  November  meteors  oc- 
cur becomes  gradually  later  and  later.     In  1866  and  1867  the 
great  display  was  November  14th ;  in  1832  and  1833  it  was  No- 
vember 13th  ;  in  1799  it  was  November  12th ;  in  1698  it  was  No- 
vember 9th ;  and  the  earliest  recorded  corresponding  displays  oc- 
curred in  October.     If  we  suppose  that  these  meteors,  before  they 
encounter  the  earth,  form  a  ring,  or  a  portion  of  a  ring,  about  the 
sun,  then  we  must  conclude  that  the  node  of  this  ring  has  a  direct 
motion  along  the  ecliptic  amounting  to  102  seconds  annually 
with  respect  to  a  fixed  equinox. 


SHOOTING-STARS,  METEORS,  AND  AEROLITES.  233 

458.  Period  of  the  November  Meteors. — A  comparison  of  the  dates 
mentioned  in  Art.  454  shows  that  the  grand  displays  recur  after  a 
cycle  of  about  one  third  of  a  century,  and  that  a  grand  display 
may  occur  on  two  consecutive  years.  A  number  greater  than 
usual  may  be  observed  also  for  three  or  four  consecutive  years. 
Hence  we  must  conclude  that  these  meteors  belong  to  a  system 
of  small  bodies  describing  an  elliptic  orbit  about  the  sun,  and  ex- 
tending in  the  form  of  a  stream  along  a  considerable  arc  of  that 
orbit.  It  is  evident  that  the  meteors  can  not  make  more  than 
two  complete  revolutions  in  a  year,  for  the  major  axis  of  an  orbit 
which  should  be  completed  in  one  third  of  a  year  would  not  reach 
from  the  sun  to  the  earth.  Hence  we  conclude  that  in  one  year 
the  group  of  meteors  must  describe  either  2  ±^3-1  or  1—  -sVi  or  73- 
revolutions;  that  is,  the  periodic  time  must  be  either  180,  185, 
354  or  376  days,  or  33£  years. 

The  motion  of  the  node  of  a  group  of  meteors  describing  an 
orbit  about  the  sun  in  each  of  the  preceding  periods  has  been 
computed,  and  it  is  found  that  the  motion  corresponding  to  either 
of  the  first  four  mentioned  periods  would  be  entirely  incompatible 
with  the  motion  actually  observed ;  but  if  the  period  be  assumed 
33^  years,  the  computed  motion  of  the  node  due  to  the  action  of 
the  planets  agrees  almost  exactly  with  the  observed  motion. 
This  coincidence  is  regarded  as  demonstrating  that  the  true  pe- 
riod of  the  November  meteors  is  33^  years. 

458.  Elements  of  the  November  Meteors. — Assuming  the  period 
as  thus  determined,  and  also  the  position  of  the  radiant  point 
shown  by  the  observations,  it  is  possible  to  compute  the  elements 
of  the  orbit.  These  elements  are  given  in  the  first  part  of  the  fol- 
lowing table  : 

November  Meteors.  Comet  of  1866. 

Period 33.25  years.  33.18  years. 

Semi-axis  major    .     .     .  10.3402  10.3248 
Eccentricity      ....       0.9047  0.9054 

Perihelion  distance    .     .       0.9855  0.9765 

Inclination 16°  46'  17°  18' 

Longitude  of  node     .     .      51°  28'  51°  26' 

Longitude  of  perihelion  .      58°  19X  60°  28' 

Motion Retrograde.  Retrograde. 

Figure  96,  p.  234,  shows  the  form  and  dimensions  of  this  orbit 


234 


METEOROLOGY. 


459.  First  Comet  c/1866. — The  elements  of  the  first  comet  of 
1866  bear  a  remarkable  resemblance  to  those  of  the  November 
meteors.     These  elements  are  given  in  the  last  column  of  the  pre- 
ceding table.     It  is  very  improbable  that  so  close  a  coincidence 
should  be  accidental,  and  hence  we  seem  authorized  to  conclude 
that  the  comet  of  1866  is  a  very  large  meteor  belonging  to  the  No- 
vember stream. 

460.  Dimensions   of  the  November   Stream.  —  The  November 
stream  of  meteors  is  several  years  in  passing  its  node.     The 
length  of  the  period  during  which  extraordinary  displays  of  me- 
teors may  occur  is  more  than  one  year,  and  an  unusual  number 
of  shooting-stars,  sufficient  to    attract   attention,  may  be   seen 
through  a  period  of  at  least  5  or  6  years.     Hence  we  conclude 
that  the  length  of  the  denser  portion  of  the  group,  when  at  peri- 
helion, is  at  least  one  fourth  of  the  circumference  of  the  orbit, 


SHOOTING-STARS,  METEORS,  AND  AEROLITES.  235 

or  one  thousand  millions  of  miles ;  while  a  large  number  of  me- 
teors extend  still  farther  along  the  orbit. 

Since  the  shower  of  1833  lasted  two  or  three  hours,  the  thick- 
ness of  the  ring  at  that  point  must  have  been  the  distance  passed 
over  by  the  earth  in  that  time,  multiplied  by  the  sine  of  the  in- 
clination of  the  orbit,  or  about  50,000  miles.  The  comet  of  1866 
passed  the  path  of  the  earth  at  a  distance  of  six  hundred  thou- 
sand miles,  which  seems  to  imply  that  the  breadth  of  the  ring  is 
much  greater  than  its  thickness. 

461.  Conclusions. — It  thus  appears  to  be  pretty  well  established 
that  the  meteors  of  November  are  derived  from  a  cosmical  cloud, 
composed  of  very  minute  elements,  each  of  which,  before  it  en- 
countered the  earth,  was  moving  in  an  elliptic  path  about  the  sun 
with  a  period  of  33£  years.     This  cloud  has  the  form  of  an  ellip- 
tic arc,  the  denser  portion  of  which  is  at  least  600  millions  of 
miles  in  extent  when  near  perihelion,  and  the  rarer  portion  ex- 
tends very  much  farther  along  the  ellipse,  while  its  thickness, 
where  greatest,  is  over  50,000  miles.     This  cloud,  although  of 
immense  extent,  has  very  small  density.      It  is  computed  that 
the  mean  distance  of  the  individual  elements  of  the  group  from 
each  other  when  near  perihelion  is  30  or  40  miles ;  and  although 
some  of  the  meteors  may  have  considerable  size,  their  weight  is 
doubtless  very  small.     Hence  the  planets  pass  freely  through 
the  densest  portion  of  this  cloud  without  any  sensible  loss  of 
motion. 

462.  Division  of  Biela's  Comet. — Admitting  that  the  November 
meteors  have  a  period  of  33J  years,  we  find  by  computation  that 
Biela's  comet  passed  extremely  near,  and  probably  through  the 
meteoric  stream  near  the  close  of  December,  1845.     It  has  been 
conjectured  that  this  collision  may  have  produced  the  separation 
of  this  comet  into  two  parts — a  separation  which  was  first  no- 
ticed December  29th. 

It  is  probable,  however,  that  the  density  of  the  stream  of  mete- 
ors at  this  point  was  extremely  small,  so  that  this  cause  would 
seem  inadequate  to  account  for  the  division  of  Biela's  comet. 

463.  The  Periodical  Meteors  of  August.  —  Another  season  at 
which  meteors  appear  each  year  in  unusual  numbers  occurs  about 


236  METEOROLOGY. 

the  10th  of  August.  The  periodicity  of  this  display  was  estab- 
lished in  1837,  since  which  time  an  extraordinary  number  of  me- 
teors lias  been  uniformly  observed  each  year,  both  in  Europe 
and  America,  from  the  6th  to  the  13th  of  the  month,  the  greatest 
number  being  generally  seen  on  the  morning  of  the  10th.  At 
the  time  of  the  maximum,  the  number  of  meteors  visible  is  about 
three  times  as  great  as  for  the  average  of  the  entire  month,  and 
five  times  as  great  as  for  the  average  of  the  entire  year. 

The  meteors  of  August,  like  those  of  November,  seem  also 
to  emanate  chiefly  from  a  fixed  point  in  the  heavens.  This 
point  is  in  the  constellation  Perseus,  being  in  R.  A.  44°,  and 
Dec.  56°  N. 

464.  Elements  of  the  Orbit  of  the  August  Meteors. — Assuming 
that  the  meteors  radiated  from  the  point  just  stated;  that  the  or- 
bit is  a  parabola,  and  that  the  earth  crossed  the  centre  of  the 
group  in  1866,  Aug.  10.75,  the  elements  given  in  the  first  part  of 
the  following  table  have  been  computed: 

Angus.  Meteors.          Third  Comet  of  1862. 

Longitude  of  perihelion  .    343°  28'  344°  41' 

Longitude  of  node     .     .    138°  16'  137°  27' 

Inclination 64°    3'  66°  26' 

Perihelion  distance    .     .      0.9643  0.9626 

Period 121.5  years. 

Motion Retrograde.          Retrograde. 

The  elements  of  the  third  comet  of  1862,  given  in  the  last  col- 
umn of  the  preceding  table,  bear  a  remarkable  resemblance  to 
those  of  the  August  meteors.  The  difference  is  no  greater  than 
can  be  accounted  for  by  the  want  of  precision  in  the  data  for  com- 
puting the  paths  of  the  meteors.  Hence  we  conclude  that  the 
great  comet  of  1862  was  one  of  the  August  meteors,  and  probably 
one  of  the  largest  of  that  group. 

465.  Dimensions  of  the  August  Stream. — It  is  considered,  then, 
highly  probable  that  the  August  meteors  describe  a  very  large 
elliptic  orbit  about  the  sun,  extending  considerably  beyond  the 
orbit  of  Neptune.     It  is  probable  that  the  meteors  are  spread 
over  the  entire  circumference  of  this  orbit,  but  not  in  equal  num- 
bers.    There  are  on  record  63  remarkable  displays  of  meteors 
which  are  considered  to  belong  to  this  group,  the  earliest  having 


SHOOTING-STABS,  METEORS,  AND  AEROLITES.  237 

occurred  A.D.  811.  A  comparison  of  these  dates  affords  some  in- 
dication of  a  maximum  of  brilliancy  recurring  at  intervals  of  108 
years. 

The  earth,  moving  at  the  rate  of  68,000  miles  per  hour,  is  at 
least  seven  days  in  passing  entirely  through  the  ring,  which  indi- 
cates that  the  thickness  of  the  ring  is  more  than  eleven  millions 
of  miles. 

The  density  of  this  stream  of  meteors  is  quite  small,  the  mean 
distance  of  the  individuals  of  the  group  from  each  other  being 
computed  to  be  more  than  a  hundred  miles. 

466.  Origin  of  Meteoric  Streams. — Streams  of  meteors  moving 
about  the  sun  in  orbits  of  vast  extent  may  be  supposed  to  have 
resulted  from  a  nebulous  mass,  or  cosmical  cloud,  acted  upon  by 
the  attraction  of  the  sun.     Let  us  suppose  a  cosmical  cloud,  con- 
sisting of  very  small  meteors,  to  be  drawn  from  stellar  space  by 
the  attraction  of  the  sun.     The  individual  particles  of  the  cloud 
will  move  in  elliptic  orbits  about  the  sun,  but  these  ellipses  will  not 
be  exactly  equal  to  each  other.     If  the  form  of  the  cloud  were  at 
first  spherical,  its  shape  would  be  gradually  changed,  and  it  would 
ultimately  be  drawn  out  into  a  parabolic  or  elliptic  arc,  of  which 
the  sun  is  the  focus.     If  the  orbit  were  an  ellipse,  the  original 
form  of  the  cloud  would  never  be  regained.     At  each  perihelion 
passage  the  length  of  the  stream  would  be  increased,  and  after  a 
certain  number  of  revolutions  the  cloud  would  become  a  con- 
tinuous ring.     The  stream  would  be  at  first  periodic,  but  finally 
the  flow  would  be  constant.     If  the  primitive  form  of  the  group 
was  not  spherical,  similar  results  would  follow.     The  meteors  of 
November  are  supposed  to  belong  to  such  a  group,  in  which  the 
ring  is  only  partially  formed,  while  the  August  meteors  repre- 
sent a  group  which  has  been  transformed  into  a  continuous  ring. 
Hence  it  is  inferred  that  the  November  group  is  of  compara- 
tively recent  formation. 

467.  Other  Periods  of  Shooting-stars. — Besides  the  months  of 
August  and  November,  there  are  several  other  periods  at  which, 
either  annually  or  occasionally,  shooting-stars  have  been  observed 
in  unusual  numbers.     Of  these,  the  best  established  periods  are 
shown  in  the  following  table,  which  also  gives  the  radiant  point 
from  which  the  meteors  seem  chiefly  to  emanate.     These  meteors 


238  METEOROLOGY. 

are  generally  found  to  have  a  pretty  definite  radiant  point,  like 
the  meteors  of  November  and  August. 

Date  of  Display.  Radiant  Point  of  the  Meteors. 

Jan.  2  .     .  A.  R.  234° ;  N.  Dec.  51°,  near  £  Cor.  Borealis. 

April  20  .      "    277°;        "       35°,    "     a  Lyree. 

July  28-29    "     304°;        "       40°,    "     7  Cygni. 

Oct.  24     .      "       83°;        "       12°,    "     a  Orionis. 

Dec.  8-13  "  105°;  "  30°,  "  T  Geminorum. 
The  meteors  which  are  seen  on  ordinary  nights,  and  which  do 
not  show  any  marked  uniformity  of  direction,  have  been  called 
sporadic.  It  is,  however,  not  improbable  that  meteors  which  at 
present  are  regarded  as  sporadic,  may  hereafter  be  proved  to  be 
periodical.  It  seems  probable  that  shooting-stars,  before  they 
encounter  the  earth,  form  in  the  planetary  spaces  a  multitude  of 
currents  or  continuous  rings,  differing  greatly  in  size  and  density, 
situated  at  various  distances  from  the  sun,  and  having  all  possible 
inclinations  to  the  ecliptic.  The  unequal  number  of  shooting- 
stars  witnessed  on  different  days  of  the  year  is  the  consequence 
of  the  unequal  distribution  of  these  meteoric  streams  throughout 
the  planetary  spaces. 

SECTION  II. 

DETONATING   METEORS. 

468.  Detonating  Meteors  defined.  —  Ordinary  shooting-stars  are 
:iot  accompanied  by  any  audible  sound,  although  they  are  some- 
times seen  to  break  into  pieces.     Occasionally  meteors  of  extra- 
ordinary brilliancy  are  succeeded  by  a  loud  detonation,  or  explo- 
sion, followed  by  a  noise  like  that  of  musketry,  or  the  discharge 
of  cannon.     These  have  been  called  detonating  meteors. 

469.  Tfie  New  Jersey  Meteor  of  November  I5lh,  1859. — On  the 
morning  of  November  15th,  1859,  about  9£  o'clock,  a  remarkable 
meteor  appeared  in  the  heavens  over  the  southern  part  of  New 
Jersey.     It  was  so  brilliant  that,  although  the  sun  was  unclouded, 
and  had  an  elevation  of  about  20°  above  the  horizon,  the  flash 
attracted  the  attention  of  multitudes  of  persons  as  far  north  as 
Albany  and  Boston,  and  as  far  south  as  Fredericksburg,  Virginia. 
Its  apparent  path  was  downward,  inclined  a  few  degrees  to  the 
west,  and  it  left  behind  it  a  cloud  of  a  rounded  form  like  a  puff 


SHOOTING-STARS,  METEORS,  AND  AEROLITES.  239 

of  smoke.  Soon  after  the  flash  there  was  heard  a  series  of  terrific 
explosions,  which  were  compared  to  the  discharge  of  a  thousand 
cannon.  These  explosions  were  heard  throughout  Delaware  and 
most  of  New  Jersey.  From  a  comparison  of  numerous  observa- 
tions, it  was  computed  that  the  height  of  this  meteor,  when  first 
seen,  was  over  60  miles,  and  when  it  exploded  its  height  was  20 
miles.  The  length  of  its  visible  path  was  more  than  40  miles. 
It  described  this  path  in  two  seconds,  so  that  its  velocity  relative 
to  the  earth  was  at  least  20  miles  per  second.  The  column  of 
smoke  resulting  from  the  explosions  was  a  thousand  feet  in  di- 
ameter, and  several  miles  in  length. 

Comparing  the  motion  of  this  meteor  with  that  of  the  earth  in 
its  orbit,  we  find  that  its  velocity  relative  to  the  sun  was  about  28 
rniles  per  second,  which  is  the  velocity  belonging  to  a  parabolic 
orbit.  The  lowest  admissible  estimate  of  its  velocity  would  indi- 
cate that  this  meteor  was  moving  about  the  sun  in  a  very  eccen- 
tric ellipse;  the  most  probable  velocity  would  indicate  that  its 
path  was  either  a  parabola  or  an  hyperbola. 

470.  The  Tennessee  Meteor  of  August  2d,  I860.— On  the  2d  of 
August,  1860,  about  10  P.M.,  a  magnificent  fire-ball  was  seen 
throughout  the  whole  region  from  Pittsburg  to  New  Orleans,  and 
from  Charleston  to  St.  Louis,  an  area  of  nine  hundred  miles  in  di- 
ameter.    It  was  described  as  equal  in  size  to  the  full  moon,  and 
before  its  disappearance  it  broke  into  several  fragments.     A  few 
minutes  after  its  disappearance,  there  was  heard  throughout  sev- 
eral counties  of  Kentucky  and  Tennessee  a  tremendous  explosion 
like  the  sound  of  distant  cannon. 

From  a  comparison  of  a  large  number  of  observations,  it  has 
been  computed  that  this  meteor,  when  first  seen,  was  about  82 
miles  above  the  earth's  surface,  and  it  exploded  at  an  elevation 
of  28  miles.  The  length  of  its  visible  path  was  about  240  miles, 
and  time  of  flight  8  seconds,  showing  a  velocity  relative  to  the 
earth  of  30  miles  per  second.  It  is  hence  computed  that  its  ve- 
locity relative  to  the  sun  was  24  miles  per  second. 

471.  Number,  Velocity,  etc. — Examples  of  detonating  meteors 
similar  to  the  preceding  are  of  yearly  occurrence,  and  if  every 
case  was  duly  reported,  they  would  probably  be  found  to  be  of 
daily  and  perhaps  hourly  occurrence.     The  number  of  detonating 


240  METEOROLOGY. 

meteors  found  recorded  in  scientific  journals  is  over  800.  Their 
average  height  at  the  first  instant  of  apparition  is  92  miles,  and  at 
the  instant  of  vanishing  is  32  miles.  Their  average  velocity  rel- 
ative to  the  earth  is  estimated  at  19  miles  per  second. 

472.  Multiple  Nuclei,  etc. — Sometimes  the  head  of  a  meteor  ap- 
pears divided,  consisting  of  two  or  more  brilliant  bodies  in  the 
form  of  elongated  drops,  each  followed  by  a  tail  of  fiery  appear- 
ance.    In  a  few  cases  as  many  as  a  dozen  heads  have  been  count- 
ed, but  generally  these  secondary  heads  follow  the  principal  body 
of  light  so  closely  that  they  give  to  the  meteor  an  elongated  ap- 
pearance, which  has  been  sometimes  compared  to  a  child's  kite,  a 
pear,  a  fish,  etc. 

The  track  of  the  meteor  is  often  marked  by  a  permanent  streak, 
which  sometimes  continues  visible  for  many  minutes.  This  streak 
gradually  changes  its  shape  and  position,  like  a  cloud  moved  by 
the  wind,  sometimes  assuming  a  serpentine  form,  sometimes  bend- 
ing up  like  a  crescent  or  a  horse-shoe,  and  drifting  with  a  velocity 
of  more  than  100  miles  per  hour. 

473.  Periodicity  of  Detonating  Meteors. — An  unusual  number  of 
detonating  meteors  has  been  seen  about  the  time  of  the  grand 
meteoric  display  of  November  18th ;  also  about  the  time  of  the 
grand  display  of  August  10th ;   and  also  December  8th -13th. 
Moreover,  several  detonating  meteors  have  been  recorded  Janua- 
ry 2d  and  April  20th.     This  coincidence  in  the  times  of  unus- 
ual display  of  detonating  meteors  and  of  ordinary  shooting-stars, 
taken  in  connection  with  the  results  obtained  respecting  their 
paths  and  velocities,  leads  us  to  infer  that  both  belong  to  the  same 
class  of  bodies,  and  that  they  do  not  probably  differ  much  from 
each  other  except  in  size  and  density.     We  conclude,  then,  that 
detonating  meteors  are  small  bodies  which  revolve  about  the  sun 
in  orbits  which  are  generally  ellipses  of  considerable  eccentricity, 
but  perhaps  sometimes  parabolas  or  even  hyperbolas.     They  are 
bodies  of  considerable  density,  and  the  noise  which  succeeds  their 
appearance  is  probably  in  great  part  due  to  the  collapse  of  the 
air  rushing  into  the  vacuum  which  is  left  behind  the  advancing 
meteor.     No  audible  sound  proceeds  from  ordinary  shooting- 
stars,  because  they  are  bodies  of  small  size  or  of  feeble  density, 
and  are  generally  dissipated  or  consumed  while  yet  at  an  eleva- 
tion of  50  miles  above  the  earth's  surface. 


SHOOTING-STARS,  METEORS,  AND  AEROLITES.  241 


SECTION  III. 

AEROLITES. 

474.  Aerolites  described. — There  is  no  evidence  that  any  deposit 
from  ordinary  shooting-stars  ever  reaches  the  earth's  surface,  but 
occasionally  solid  substances  descend  to  the  earth  from  beyond 
the  earth's  atmosphere.    The  fragments  generally  penetrate  a  foot 
or  more  into  the  earth,  and  if  picked  up  soon  after  their  fall  are 
found  to  be  warm,  and  sometimes  even  hot.     These  small  bodies 
are  called  aerolites.     They  are  called  meteoric  stones  when  they 
present  a  stony  appearance,  or  meteoric  iron  when  they  are  almost 
entirely  metallic. 

Although  numerous  instances  of  the  fall  of  aerolites  had  been 
recorded  from  the  earliest  historic  times,  and  especially  during 
the  last  century,  these  accounts  were  received  by  many  scientific 
men  with  incredulity.  But  during  the  present  century  these 
cases  have  been  so  numerous,  and  they  have  been  witnessed  by 
so  many  observers,  that  we  can  no  longer  doubt  that  stones  have 
fallen  to  the  earth  from  beyond  the  earth's  atmosphere. 

475.  The  Weston,  Connecticut,  Aerolite. — On  the  morning  of  De- 
cember 14th,  1807,  a  meteor  of  great  brilliancy  was  seen  moving 
through  the  atmosphere  over  the  town  of  Weston,  Connecticut. 
Its  apparent  diameter  was  about  one  half  that  of  the  full  moon ; 
and  soon  after  its  disappearance  there  were  heard  by  those  near 
ly  under  the  place  of  disappearance  three  loud  explosions  like 
those  of  a  cannon,  followed  by  a  quick  succession  of  smaller  re- 
ports.    Immediately  after  the  explosions,  one  observer  heard  a 
sound  like  that  occasioned  by  the  fall  of  a  heavy  body,  and,  upon 
examination,  found  that  a  stone  had  fallen  upon  a  rock  near  his 
house,  and  was  broken  into  small  fragments.    The  fragments  were 
still  warm,  and  together  were  estimated  to  weigh  about  twenty 
pounds. 

In  another  place,  about  five  miles  from  the  former,  a  fresh  hole 
was  found  in  the  turf,  and  at  the  bottom  of  the  hole,  at  the  depth 
of  two  feet,  was  found  a  stone  weighing  thirty-five  pounds.  IE 
the  neighborhood  was  found  a  third  stone  weighing  about  ten 
pounds,  a  fourth  weighing  thirteen  pounds,  a  fifth  weighing  twen- 
ty pounds,  and  a  sixth  weighing  thirty-six  pounds.  At  a  spot 

Q 


242  METEOROLOGY. 

about  four  miles  distant  from  the  preceding,  a  large  mass  of 
stones,  estimated  to  weigh  200  pounds,  fell  upon  a  rock,  and  was 
broken  into  minute  fragments.  It  was  estimated  that  the  entire 
weight  of  all  the  fragments  was  at  least  300  pounds. 

The  specimens  from  all  these  localities  were  quite  similar,  and 
their  specific  gravity  varied  from  3.3  to  3.6.  Their  composition 
was  nearly  one  half  silex,  about  one  third  oxyd  of  iron,  and  one 
sixth  magnesia,  with  a  little  nickel  and  sulphur. 

The  same  meteor  was  extensively  seen  as  far  north  as  Ver- 
mont, and  as  far  south  as  New  Jersey.  The  length  of  its  visible 
path  exceeded  100  miles,  and  it  moved  from  northwest  to  south- 
east, its  path  being  inclined  downward  about  30°  to  the  horizon, 
and  when  it  exploded  its  elevation  was  only  about  eight  miles. 
The  time  of  flight  was  probably  between  five  and  ten  seconds. 
Hence  the  velocity  relative  to  the  earth  was  about  fifteen  miles 
per  second. 

476.  The  Guernsey^  Ohio,  Aerolite. — On  the  first  of  May,  1860, 
about  half  an  hour  after  noon,  an  aerolite  exploded  over  Guern- 
sey County,  Ohio.     A  great  number  of  distinct  detonations  were 
heard,  like  the  firing  of  a  cannon,  after  which  the  sounds  became 
blended  together,  and  were  compared  to  the  roar  of  a  railway 
train.    The  elevation  of  this  meteor  above  the  earth's  surface  was 
computed  at  forty-one  miles,  and  its  path  was  nearly  horizontal. 
The  entire  weight  of  all  the  fragments  which  descended  from  this 
meteor  was  estimated  at  700  pounds.     Their  specific  gravity  was 
3.54,  and  their  composition  very  similar  to  that  of  the  Weston 
meteor. 

477.  The  Braunau,  Bohemia,  Aerolite. — On  the  14th  of  July, 
1847,  about  four  o'clock  in  the  morning,  at  Braunau,  in  Bohemia, 
there  were  heard  two  heavy  explosions,  which  followed  each  oth- 
er in  quick  succession.     Two  streams  of  fire  were  seen  to  descend 
to  the  earth,  and,  upon  examination,  a  fresh  hole  three  feet  deep 
was  found  in  the  earth,  and  at  the  bottom  of  the  hole  a  mass  of 
iron,  which  for  six  hours  after  the  fall  continued  so  hot  that  it 
could  not  be  held  in  the  hand.     This  mass  weighed  forty -two 
pounds,  and  is  preserved  in  the  cabinet  at  Vienna.     Another 
mass,  weighing  thirty  pounds,  fell  upon  a  roof,  and  broke  through 
large  pieces  of  timber. 


SHOOTING-STARS,  METEORS,  AND   AEROLITES.  243 

The  specific  gravity  of  this  meteor  was  7.71.  Its  composition 
was  ninety-two  per  cent,  of  iron,  and  five  per  cent,  of  nickel,  with 
a  small  quantity  of  cobalt,  arsenic,  etc. 

478.  The  Orgueil,  France,  Aerolite.  —  On  the  evening  of  May 
14th,  1864,  a  very  bright  fire-ball  was  seen  in  France,  throughout 
the  whole  region  from  Paris  to  the  Pyrenees.     Loud  detonations 
were  heard  in  the  neighborhood  of  Montauban,  and  a  large  num- 
ber of  stones  fell  near  the  village  of  Orgueil.     The  passage  of  the 
meteor  was  witnessed  by  a  large  number  of  intelligent  observers. 
It  was  first  seen  at  an  altitude  greater  than  fifty-five  miles;  it  ex- 
ploded at  an  altitude  of  about  twent}'  miles ;  and  it  was  descend- 
ing in  a  line  inclined  20C  or  25°  to  the  horizon.     The  length  of 
its  visible  path  was  112  miles;  and  the  time  of  flight  was  esti- 
mated at  five  or  six  seconds,  indicating  a  velocity  of  not  less  than 
fifteen  or  twenty  miles  per  second.     The  stones  were  hot  when 
they  were  first  picked  up.     Their  specific  gravity  was  2.567. 

479.  Number  of  Aerolites. — There  are  thirty  well-authenticated 
cases  in  which  aerolites  have  fallen  in  the  United  States  dur- 
ing the  last  eight}''  years,  and  their  aggregate  weight  is  2690 
pounds.    The  entire  number  of  known  aerolites,  the  date  of  whose 
fall  is  well  determined,  is  261.     There  are  also  on  record  seventy- 
four  cases  of  aerolites  in  which  the  day  and  month  are  not  given, 
and  sometimes  even  the  year  is  uncertain.     Besides  these  there 
have  been  found  eighty -six  masses,  which,  from  their  peculiar 
composition,  are  believed  to  be  aerolites,  although  the  date  of 
their  fall  is  unknown.     The  weight  of  these  masses  varies  from 
a  few  pounds  to  several  tons.     The  entire  number  of  aerolites  of 
which  we  have  any  knowledge  is  therefore  about  420. 

The  actual  number  of  aerolites  which  have  reached  the  earth 
;nust  have  been  far  greater  than  this.  Many  must  have  fallen 
upon  the  ocean,  or  upon  uninhabited  lands  where  they  were  un- 
observed. During  the  past  fifty  years  the  fall  of  115  aeroliteg 
has  been  recorded.  If  we  suppose  aerolites  to  have  fallen  over 
the  entire  globe  at  the  same  rate  as  has  been  observed  ever  tbc 
more  populous  portions  of  Europe  and  America,  we  should  have 
an  average  of  over  300  annually.  Now  we  can  not  suppose  that 
even  in  Europe  more  than  half  the  whole  number  are  actually 
seen  to  fall;  hence  we  conclude  that  more  than  600  aerolites  fall 


244 


METEOROLOGY. 


annually  on  various  parts  of  the  earth's  surface.  If  we  suppose 
their  average  weight  to  equal  that  of  those  which  have  fallen  in 
the  United  States,  we  should  have  for  the  entire  globe  eighteen 
tons  of  aerolites  annually.  See  Tables  XXXV.  and  XXXVI. 

480.  Chemical  Composition  of  Aerolites. — Aerolites  are  composed 
of  the  same  elementary  substances  as  occur  in  terrestrial  minerals, 
not  a  single  new  element  having  been  found  in  their  analysis. 
Of  the  sixty-three  elements  now  admitted  by  chemists,  the  follow- 
ing twenty  or  twenty-two  have  been  found  in  aerolites. 


Me 

1.  Aluminium. 
2.  Calcium. 
3.  Chromium. 
4.  Cobalt. 

als. 

9.  Manganese. 
10.  Nickel. 
11.  Potassium. 
12.  Sodium. 

Metalloids. 

1.  Carbon. 
2.  Oxygen. 
3.  Phosphorus. 
4.  Silicium. 

5.  Copper. 
6.  Iron. 

13.  Strontium. 
14  Tin. 

5.  Sulphur. 
6.  Arsenic? 

7.  Lithium. 

15.  Titanium. 

7.  Chlorine? 

8.  Magnesium. 

Aerolites  differ  greatly  in  the  proportions  of  these  ingredients. 
Some  of  them  contain  ninety -six  per  cent,  of  iron,  while  others 
contain  less  than  one  per  cent.  Some  contain  eighteen  per  cent, 
of  nickel,  and  others  less  than  one  per  cent.  On  the  contrary, 
others  consist  mostly  of  silica,  magnesia,  lime,  etc.  It  is  common, 
therefore,  to  divide  aerolites  into  two  groups,  viz.,  meteoric  iron 
and  meteoric  stones. 

The  specific  gravity  of  aerolites  varies  from  1.70  (that  of  March 
15th,  1806,  at  Alais,  France)  to  7.8  (that  of  May  26th,  1751,  at 
Agram,  in  Austria). 

481.  Peculiarities  of  Aerolites. — While  aerolites  contain  no  ele- 
ments but  such  as  are  found  in  terrestrial  minerals,  their  appear- 
ance is  quite  peculiar,  and  the  grouping  of  the  elements,  that  is, 
the  compounds  formed  by  them,  are  so  peculiar  as  to  enable  us 
by  chemical  analysis  to  distinguish  an  aerolite  from  any  terres- 
trial substance. 

Iron  ores  are  very  abundant  in  nature,  but  iron  in  the  metallic 
state  is  exceedingly  rare  in  nature.  Now  aerolites  invariably 
contain  metallic  iron,  sometimes  ninety  to  ninety -six  per  cent. 


SHOOTING-STARS,  METEORS,  AND   AEROLITES.  245 

This  iron  is  perfectly  malleable,  and  may  be  readily  worked  into 
cutting  instruments.  This  meteoric  iron  always  contains  a  cer- 
tain amount  of  nickel,  generally  eight  or  ten  per  cent.,  with  small 
quantities  of  cobalt,  copper,  tin,  and  chrome.  This  composition 
has  never  been  found  in  any  terrestrial  mineral.  Moreover,  when 
the  fragments  of  meteoric  iron  which  are  dispersed  through  those 
aerolites  which  are  mostly  earthy  are  extracted  and  submitted  to 
analysis,  they  show  the  same  composition,  viz.,  about  ninety  of 
iron,  with  eight  or  ten  of  nickel,  etc. 

Many  of  the  other  constituents  of  aerolites  are  similar  to  those 
which  are  found  in  volcanic  rocks,  such  as  olivine  (a  silicate  of 
magnesia),  magnetic  pyrites,  chrome-iron,  etc. 

All  aerolites,  without  exception,  contain  a  substance  called 
schreibersite,  though  often  in  very  small  quantities.  This  sub- 
Btance  is  a  compound  of  iron,  nickel,  and  phosphorus,  and  has 
never  been  found  except  in  aerolites.  Fig.  97  represents  an  iron 
meteor  found  near  Lockport,  New  York,  in  1818. 

Fig.  97. 


482.  Widmannstdlen  Figures. — Meteoric  iron  possesses  a  highly 
crystalline  structure.  If  the  surface  be  carefully  polished,  and 
the  mass  be  heated  to  a  straw-yellow,  after  cooling,  the  surface 
will  be  covered  with  groups  of  regular  triangles  formed  by  lines 
nearly  parallel  to  each  other,  intersected  by  others  at  angles  of 
sixty  degrees.  These  figures  were  first  discovered  by  an  Aus- 
trian iron -master,  Widmannstaten,  in  the  year  1808,  and  they 
have  received  the  name  of  their  discoverer. 

It  was  afterward  discovered  that  the  same  figures  could  be  de- 
veloped by  the  use  of  acids.  For  this  purpose,  nitric  acid  is  di- 
luted with  an  equal  volume  of  water,  and  the  iron,  having  been 
previously  cut  and  polished,  is  placed  in  the  solution,  the  parts 
not  required  to  be  acted  upon  being  coated  with  asphaltum. 


246 


METEOROLOGY. 


After  five  or  six  minutes  the  iron  is  taken  out  of  the  acid,  care- 
fully washed  and  dried.  Figure  98  shows  the  crystalline  struc- 
ture of  the  meteoric  iron  of  Elbogen,  preserved  in  the  cabinet  of 
Vienna. 


Ordinary  iron  will  not  exhibit  these  Widmannstaten  figures, 
but  iron  melted  directly  out  of  some  volcanic  rocks  does  exhibit 
them. 

483.  Periodicity  of  Aerolites. — The  falls  of  aerolites  exhibit  some 
indications  of  periodicity,  and  these  periods  correspond  with  those 
of  ordinary  shooting-stars.  There  are  on  record  eleven  cases  in 
which  aerolites  have  been  seen  to  fall  near  the  time  of  the  annual 
display  of  the  August  meteors,  Art.  463 ;  that  is,  four  per  cent,  of 
all  the  recorded  aerolite  falls  have  occurred  within  three  days  of 
the  maximum  display  of  August  meteors.  This  number  is  more 
than  double  that  which  we  should  expect  if  aerolites  and  shoot- 
ing-stars had  no  connection  with  each  other. 

There  are  on  record  seven  cases  in  which  aerolites  have  fallen 
between  December  7th  and  13th,  which  is  also  a  period  of  unus- 
ual display  of  shooting-stars,  Art.  467  ;  and  there  are  also  three' 
cases  in  which  aerolites  have  fallen  from  November  llth  to  13th. 
These  numbers  are  greater  than  should  be  expected  if  shooting- 
stars  and  aerolites  were  entirely  independent  of  each  other. 

It  is  not  probable  that  such  a  coincidence  of  dates  is  accidental, 


SHOOTING-STARS,  METEORS,  AND  AEROLITES.  247 

and  hence  we  are  led  to  conclude  that  aerolites  form  portions  of 
the  nebulous  rings  or  groups  from  which  shooting-stars  are  de- 
rived. 

483.  Are  Aerolites  formed  in  our  Atmosphere? — Various  hypoth 
eses  have  been  proposed  to  account  for  the  origin  of  aerolites.    It 
has  been  conjectured  that  they  are  formed  in  our  atmosphere  like 
rain  or  hail.     This  supposition  is  inadmissible,  because,  allowing 
the  aerolite  to  be  once  formed,  there  is  no  known  force  which 
could  impel  it  in  a  direction  nearly  horizontal  with  a  velocity  of 
several  miles  per  second. 

484.  Have  Aerolites  been  ejected  from  Terrestrial  Volcanoes? — It 
has  been  conjectured  that  aerolites  are  masses  ejected  from  terres- 
trial volcanoes.     This  supposition  is  inadmissible,  because  the 
greatest  velocity  with  which  stones  have  ever  been  ejected  from 
volcanoes  is  less  than  two  miles  per  second,  and  the  direction  of 
this  motion   must  be  nearly  vertical,  while  aerolites  frequently 
move  in  a  direction  nearly  horizontal,  and  with  a  velocity  of  sev- 
eral miles  per  second.    This  argument  is  unanswerable,  and  there- 
fore it  is  superfluous  to  add  that  the  composition  of  aerolites  is 
different  from  that  of  any  known  terrestrial  mineral. 

485.  Have  Aerolites  been  ejected  from  Lunar  Volcanoes? — It  has 
been  conjectured  that  aerolites  have  been  ejected  from  volcanoes 
in  the  moon  with  a  velocity  sufficient  to  carry  them  out  of  the 
sphere  of  the  moon's  attraction  into  that  of  the  earth's  attraction. 
It  has  been  computed  that  a  velocity  of  projection  of  8000  feet 
per  second  would  be  sufficient  to  produce  such  an  effect. 

The  following  are  some  of  the  objections  to  this  hypothesis: 
1.  In  order  that  a  body  projected  from  the  moon  may  reach  the 
earth's  surface,  it  must  describe  about  the  earth  a  conic  section 
whose  distance  at  perigee  is  less  than  the  earth's  radius.  Hence 
there  are  limits  to  the  direction  in  which  the  aerolite  must  have 
left  the  moon,  and  also  to  the  force  with  which  it  must  have  been 
projected.  If  a  body  was  projected  from  near  the  moon's  centre, 
or  from  its  eastern  hemisphere,  since  it  would  retain  the  moon's 
orbital  velocity,  its  resulting  velocity  would  be  such  that  its  peri- 
gee distance  would  exceed  4000  miles.  If  the  body  was  project- 
ed with  a  small  force,  it  would  not  get  beyond  the  sphere  of  the 


248  METEOROLOGY. 

moon's  attraction  ;  and  if  the  velocity  was  too  great,  the  perigee 
distance  would  exceed  4000  miles.  It  has  been  computed  that  a 
change  of  T-g-0-  part  in  the  force  of  projection  would  cause  a  change 
of  more  than  4000  miles  in  the  perigee  distance,  and  for  a  given 
force  of  projection  a  change  of  -1^-5-  part  in  the  mass  of  the  body 
would  produce  a  like  effect. 

Hence  it  has  been  estimated  that  if  an  indefinite  number  of 
bodies,  having  different  masses,  were  expelled  from  the  moon  in 
all  directions  and  with  different  velocities,  not  one  in  a  million 
could  reach  the  earth.  But  it  is  computed  that  600  aerolites  fall 
to  the  earth  annually,  Art.  479.  Hence  the  lunar  hypothesis  re- 
quires us  to  conclude  that  more  than  600  millions  of  aerolites  are 
annually  expelled  from  the  moon.  But  the  lunar  volcanoes  are 
to  all  appearance  nearly,  if  not  entirely,  extinct ;  and  although  the 
moon  has  long  been  carefully  watched  with  the  most  powerful 
telescopes,  in  only  one  or  two  instances  have  astronomers  sus- 
pected that  they  had  discovered  any  indications  of  change.  We 
can  not,  therefore,  admit  that  lunar  volcanoes  have  ejected  rocks 
in  such  quantities  as  to  account  for  the  known  aerolites. 

2.  The  observed  velocities  of  some  aerolites  are  incompatible 
with  the  theory  that  they  are  satellites  of  the  earth.     In  order 
that  a  body  may  revolve  around  the  earth,  its  velocity  must  not 
be  less  than  5  miles,  nor  greater  than  7  miles  per  second.     If 
the  velocity  were  less  than  5,  the  body  would  fall  to  the  earth ; 
and  if  the  velocity  was  greater  than  7,  the  body  would  recede 
from  the  :.arth,  never  to  return.     Now  the  velocity  of  the  Orgueil 
meteor,  Art.  478,  certainly  exceeded  7  miles  per  second,  and  there- 
fore it  was  not  a  satellite  to  the  earth.     There  are  but  few  cases 
in  which  the  velocity  of  aerolites  has  been  even  rudely  deter- 
mined ;  but  detonating  meteors  seem  to  have  the  same  origin  as 
aerolites,  and  the  average  velocity  of  detonating  meteors  is  cer- 
tainly greater  than  7  miles  per  second. 

3.  Aerolites  appear  to  be  subject  to  a  periodicity  depending 
upon  the  season  of  the  year,  which  shows  that  they  are  satellites 
of  the  sun  and  not  of  the  earth. 

Although,  then,  we  can  not  pronounce  it  impossible  that  a 
small  bod}'-  projected  from  a  lunar  volcano  may  occasionally  have 
fallen  to  the  earth,  it  is  certain  that  aerolites  generally  can  not 
have  had  this  origin,  and  there  is  no  reason  to  suppose  that  any 
aerolite  has  ever  been  derived  from  this  source. 


SHOOTING-STARS,  METEORS,  AND  AEROLITES.  249 

486.  Conclusions. — A  comparison  of  all  the  facts  which  are 
known  respecting  shooting-stars,  detonating  meteors,  and  aerolites 
leads  to  the  conclusion  that  they  are  all  minute  bodies  revolving 
like  the  comets  in  orbits  about  the  sun,  and  are  encountered  by 
the  earth  in  its  orbital  motion.  The  visible  path  of  aerolites  is 
somewhat  nearer  to  the  earth's  surface  than  that  of  ordinary 
shooting-stars,  a  result  which  may  be  ascribed  to  their  greater 
density.  It  is  probable,  also,  that  their  velocity  is  somewhat 
smaller,  a  result  which  may  be  due  to  their  descending  into  an 
atmosphere  of  greater  density,  which  causes,  therefore,  greater  re- 
sistance. 

These  three  classes  of  bodies  exhibit  alternate  periods  of  maxi- 
mum and  minimum  abundance,  and  the  times  of  maximum  for 
the  several  classes  correspond  somewhat  with  each  other,  indicat- 
ing that  these  bodies  are  collected  in  groups,  and  the  three  classes 
of  bodies  are  grouped  in  a  somewhat  similar  manner.  The  Au- 
gust meteors  move  in  orbits  which  require  more  than  a  century 
to  complete,  and  comprehend  bodies  differing  greatly  in  size  and 
probably  also  in  density.  Their  magnitudes  range  from  comets 
whose  diameter  is  perhaps  100,000  miles  to  minute  atoms  which, 
in  a  single  second,  are  dissipated  by  the  heat  resulting  from  their 
collision  with  our  atmosphere.  Their  density  ranges  from  that  of 
metallic  iron  to  earthy  bodies  having  but  feeble  cohesion,  which 
are  dissipated  into  fine  dust  by  the  heat  of  collision  with  our  at- 
mosphere; and  it  is  possible  that  the  rarest  of  them  may  consist 
of  solid  or  liquid  matter  in  a  state  of  minute  subdivision,  like  a 
cloud  of  dust  or  smoke. 

The  periodic  meteors  of  November  probably  comprehend  bod- 
ies having  an  equal  range  of  magnitude,  and  perhaps  also  of  den- 
sity. 


TABLE   I. — MILLIMETRES  CONVERTED   INTO   INCHES.       251 


TABLE  I. 

TO   CONVERT  MILLIMETRES  INTO  ENGLISH  INCHES. 


Mil- 
lime- 
tres. 

Inches. 

Mil- 
lime- 
tres. 

Inches. 

Mil- 
lime- 
tres. 

Inches. 

Mil- 
lime- 
tres. 

Inches. 

Mil- 
lime- 
tres. 

Inches. 

Mil- 
lime- 
tres. 

Inches. 

I 

0.089 

5o 

1.969 

5oo 

19.  685 

689 

27.  126 

734 

28.898 

779 

3o.67o 

2 

.079 

60 

2.362 

5io 

20.079 

690 

.166 

735 

.g38 

780 

.709 

3 

,1x8 

70 

2.756 

52O 

20.4?3 

691 

.205 

736 

•977 

781 

•749 

4 

.i57 

80 

3.i5o 

53o 

20.867 

692 

.245 

737 

29.016 

782 

.788 

5 

.197 

90 

3.543 

54o 

21  .260 

693 

.284 

?38 

.o56 

783 

.827 

6 

.286 

IOO 

3.937 

55o 

21.654 

694 

.323 

739 

.ogS 

?84 

.867 

7 

.276 

I  IO 

4.33i 

56o 

22.O48 

695 

.363 

74o 

.i34 

785 

.906 

8 

.3x5 

120 

4.724 

570 

22  ,44l 

696 

.402 

74i 

.174 

786 

•  945 

9 

.354 

i3o 

5.ii8 

58o 

22.835 

697 

•  44i 

742 

.213 

787 

.gSS 

10 

•  394 

i4o 

5.5i2 

5go 

23.229 

698 

.481 

743 

.252 

788 

3i  .024 

1  1 

.433 

i5o 

5.906 

600 

23.622 

699 

.520 

?44 

.292 

789 

.o64 

12 

.472 

160 

6.299 

610 

24.016 

700 

.56o 

?45 

.33i 

79° 

.  io3 

i3 

.  5l2 

170 

6.693 

620 

24.4lO 

701 

.599 

746 

.37i 

791 

.  1  42 

i4 

.55i 

180 

7.087 

63o 

24.8o4 

702 

.638 

74? 

•  4io 

792 

.182 

i5 

.Sgi 

190 

7.480 

64o 

25.  197 

708 

.678 

748 

•  449 

793 

.221 

16 

.63o 

200 

7.874 

65o 

25.591 

704 

.717 

749 

•  489 

794 

.260 

17 

.669 

2IO 

8.268 

660 

25.985 

7o5 

.756 

?5o 

.528 

795 

.3oo 

18 

.709 

22O 

8.662 

661 

26.024 

706 

.796 

75i 

.567 

796 

.339 

'9 

•  748 

280 

g.oSS 

662 

.o63 

707 

.835 

?52 

.6o7 

797 

.379 

20 

.787 

240 

9.449 

663 

.io3 

708 

.875 

753 

.646 

798 

.418 

21 

.827 

25o 

9.843 

664 

.142 

709 

.914 

754 

.686 

799 

•  457 

22 

.866 

260 

10.  236 

665 

.182 

710 

.953 

755 

.725 

800 

•497 

23 

,go5 

270 

io.63o 

666 

.221 

711 

.993 

756 

.764 

810 

.890 

24 

•  945 

280 

i  i  .024 

667 

.260 

712 

28.032 

757 

.804 

820 

32.284 

r 

•  9^4 

290 

n.4i8 

668 

.3oo 

7i3 

.O7I 

?58 

.843 

83o 

.678 

26 

i  .024 

3oo 

11.811 

669 

.339 

?i4 

.III 

759 

.882 

84o 

33.o72 

37 

.o63 

3io 

12.  2O5 

670 

.378 

7i5 

.  i5o 

760 

.922 

85o 

.465 

28 

.  102 

32O 

12.599 

671 

.4i8 

716 

.189 

761 

.961 

860 

.85g 

29 

.  l42 

33o 

12  .992 

672 

•  457 

717 

.229 

762 

So.ooi 

87o 

34.253 

3o 

.l8l 

34o 

i3.386 

673 

•497 

718 

.268 

763 

.o4o 

880 

.646 

3i 

.220 

35o 

13.780 

674 

.536 

719 

.3o8 

764 

.079 

800 

35.o4o 

82 

O  O 

.260 

36o 

I4.I73 

675 

.575 

720 

.34? 

765 

§Ili 

900 

•  434 

o3 

.299 

370 

14.567 

676 

.6i5 

721 

.386 

766 

.i58 

Proportional 

34 

.339 

38o 

14.961 

677 

.654 

722 

.426 

767 

.197 

Parts. 

35 

.378 

3go 

i5.355 

678 

.693 

728 

.465 

768 

.287 

Mill. 

Inches. 

36 

.417 

4oo 

i5.748 

679 

.733 

724 

.5o4 

769 

.276 

0.  I 

o.oo4 

3? 

•  457 

4io 

16.  142 

680 

.772 

725 

.544 

77° 

.3:6 

O.  2 

0.008 

38 

.496 

420 

16.536 

681 

.812 

726 

.583 

771 

.355 

o.3 

O.OI2 

39 

.535 

43o 

16.929 

682 

.85i 

727 

.623 

772 

.394 

0.4 

0.016 

4o 

.575 

44o 

17.323 

683 

.890 

728 

.662 

773 

.434 

o.5 

O.O2O 

4i 

.6i4 

45o 

17.717 

684 

.980 

729 

.7OI 

774 

•  4?3 

0.6 

0.024 

42 

.654 

46o 

18.111 

685 

.969 

73o 

•  74i 

775 

.512 

0.7 

0.028 

43 

.693 

470 

i8.5o4 

686 

27.008 

73i 

.780 

776 

.552 

0.8 

o.oSr 

44 

.782 

48o 

18.898 

687 

.o48 

732 

.819 

777 

.  5gi 

0.9 

o.o35 

45 

.772 

490 

19.  292 

688 

.087 

733 

.859 

778 

.63o 

I  .0 

0.089  i 

One  millimetre  equals  0.08937079  English  inch. 


252 


TABLE  II. — METRES  CONVERTED  INTO  FEET. 


TABLE   II. 

TO  CONVERT  METRES   INTO   ENGLISH  FEET. 


Me- 
tres. 

Feet. 

Me- 
tres. 

Feet. 

Me- 
tres. 

Feet. 

Me- 

tren. 

Feet. 

Me- 
tre*. 

Feet. 

Me- 
tres. 

1 
Fe«t. 

I 

3.2S 

46 

1  50.92 

91 

298.56 

1  36 

44b.au 

181  |5y3.»4 

220 

741-48 

2 

6.56 

4? 

l54-20 

92 

3oi.84 

i37 

449-48 

182 

697.18 

227 

744.76 

3 

9-84 

48 

i57.48 

93 

3o5.  12 

i38 

452.76 

i83 

600.  4o 

228 

748.o5 

4 

l3.  12 

49 

160.76 

94 

3o8.4o 

i3g 

456.o4 

1  84 

603.69 

229 

75i.33 

5 

i6.4o 

5o 

i64-o4 

95 

3i  i  .69 

i4o 

459.33 

1  85 

606.97 

23o 

754.6i 

6 

19.69 

5i 

167.33 

96 

3i4-97 

i4i 

462.61 

186 

610.26 

23l 

757.89 

7 

22.97 

52 

170.61 

97 

3i8.25 

l42 

465.  89 

187 

6i3.53 

232 

761.17 

8 

26.26 

53 

173.89 

98 

321.53 

i43 

469.17 

1  88 

616.81 

233 

764.45 

9 

29.53 

54 

177.17 

99 

324.8i 

1  44 

472.45 

189  [620.09 

234 

767.73 

to 

32.  81 

55 

i8o.45 

100 

328.09 

i45 

4?5.73 

190 

623.37 

235 

771.01 

11 

36.09 

56 

i83.73 

101 

33i.37 

i46 

479.01 

191 

626.65 

286 

774.29 

12 

39.37 

57 

187.01 

102 

334.65 

i4? 

482.29 

192 

629.93 

237 

777.57 

i3 

42.65 

58 

190.29 

io3 

337.93 

i48 

485.  57 

i93 

633.21 

238 

78o.85 

i4 

45.  93 

59 

193.57 

io4 

34  1  .21 

i49 

488.85 

194 

636.4^ 

239 

784.i3 

i5 

49-21 

60 

196.85 

io5 

344.49 

i5o 

492  .  i  3 

195 

639.7b 

24o 

787.42 

16 

52.49 

61 

200.  i  3 

106 

347.7» 

i5i 

495.42 

196 

643.o6 

24  I 

790.70 

'7 

55.78 

62 

203.42 

107 

35i.o6 

I  52 

498.70 

197 

646.34 

242 

793.98 

18 

59.06 

63 

206.70 

108 

354.34 

i53 

5oi  .98 

198 

649.62 

243 

797.26 

'9 

62.34 

64 

209.98 

109 

357.62 

1  54 

5o5.26 

199 

652  .90 

244 

8oo.54 

20 

65.62 

65 

2l3.26 

IIO 

860.90 

i55 

5o8.54 

200 

656.18 

245 

8o3.8a 

21 

68.90 

66 

216.54 

Hi 

364.i8 

i56 

611.82 

201 

669.46 

246 

807.  10 

22 

72.  18 

67 

219.82 

112 

367.46 

i57 

5i5.  10 

2O2 

662.74 

247 

8io.38 

23 

75.46 

68 

223.  IO 

u3 

370.74 

1  58 

5i8.38 

203 

666.02 

24b 

8i3.66 

24 

78.74 

69 

226.38 

u4 

374.02 

169 

521.66 

2O4 

669  .  3o 

249 

816.94 

25 

82.02 

7° 

229.66 

u5 

377.30 

160 

524.94 

205 

672.58 

25o 

820.22 

26 

85  3o 

71 

232.94 

116 

38o.58 

161 

528.22 

206 

675.87 

25  I 

823.  5i 

27 

88.58 

72 

236.22 

117 

383.  87 

162 

53i.5i 

207 

679.  i5 

262 

826.79 

28 

91.87 

73 

239.5l 

118 

887.15 

i63 

534-79 

208 

682.43 

253 

83o.o7 

29 

95  .  i5 

74 

242  .79 

119 

390.43 

1  64 

538.07 

209 

685.  71 

254 

833.35 

3o 

98.43 

75 

246.O7 

I2O 

393.  71 

i65 

54i.35 

2IO 

688.99 

255 

836.63 

3i 

101  .71 

76 

249.35 

121 

396.99 

1  66 

544-63 

21  I 

692.27 

256 

839.91 

32 

104.99 

77 

252.63 

122 

400.27 

167 

547.91 

212 

695.55 

257 

843.ig 

33 

108.27 

78 

255.91 

123 

4o3.55 

1  68 

55i  .  19 

2l3 

698.83 

Proportional 

34 

1  1  1.  55 

79 

269.  19 

124 

4o6.83 

169 

554.4? 

2l4 

702.  i  i 

Part?. 

35 

u4.83 

80 

262  .47 

125 

4io.  i  i 

170 

557.75 

2l5 

7o5.39 

Met. 

Feet. 

36 

ii8.ii 

81 

265.75 

126 

4i3.39 

171 

56i.o3 

216 

708.67 

0.  1 

o.33 

3? 

121  .3g 

82 

269.03 

127 

416.67 

172 

564.3i 

217 

71  i  .96 

O.  2 

0.66 

38 

124.67 

83 

272.31 

128 

419.96 

178 

567.60 

218 

7i5.24 

o.3 

0.98 

39 

127.96 

84 

275.60 

129 

423.24 

i74 

570.88 

219 

718.52 

o.4 

i.3i 

4o 

i3i.24 

85 

278.88 

i3o 

426.52 

i75 

574.16 

22O 

721  .80 

o.5 

i.64 

4i 

i34.52 

86 

282.16 

i3i 

429  .80 

176 

577.44 

221 

726  .08 

0.6 

1.97 

42 

137.80 

87 

285.44 

132 

433.o8 

177 

580.72 

222 

728.36 

0.7 

2.30 

43 

i4i.o8 

88 

288.72 

i33 

436.36 

178 

584.oo 

223 

73i.64 

0.8 

2  .62 

44 

i44.36 

89 

292  .00 

1  34 

439.64 

179 

587.28 

224 

734.92 

0.9 

2  .gS 

45 

147.64 

90 

295.28 

i35 

442.92 

1  80 

690.66 

225 

738.20 

I  .0 

3.28 

One  metre  equals  3.2808992  English  feet. 


TABLE   III. — KILOMETRES  CONVERTED  INTO   MILES.        253 


TABLE   III. 

TO   CONVERT  KILOMETRES  INTO  ENGLISH  MILES. 


Kil- 
ome- 
tres. 

Miles. 

Kil- 
ome- 
tres. 

Miles. 

Kil- 
ome- 
tres. 

Miles. 

Kil- 
ome- 
tres . 

Miles. 

Kil- 
ome- 
tres. 

Miles. 

Kil- 
ome- 
tres. 

Miles. 

I 

0.621 

46 

28.584 

91 

56.546 

i36 

84-5o8 

181 

II2.47O 

226 

l4o.432 

2 

1.243 

47 

29.205 

92 

57.  167 

i37 

85.129 

182 

113.092 

227 

i4i  .o54 

3 

1.864 

48 

29.826 

93 

57.789 

1  38 

85.75i 

i83 

113.713 

228 

i4i  .675 

4 

2.486 

49 

3o.448 

94 

58.4io 

1  3g 

86.372 

1  84 

ii4.334 

22g 

142.297 

5 

3.  107 

5o 

3i  .069 

95 

5g.o3i 

i4o 

86.994 

i85 

II4.956 

23o 

142.918 

6 

3.728 

5i 

31.691 

96 

59.653 

i4i 

87.6i5 

186 

115.977 

23l 

i43.539 

7 

4.35o 

52 

32.312 

97 

60.274 

i4- 

88.236 

187 

116.  198 

232 

i44.i6i 

8 

4.971 

53 

32.933 

98 

60.895 

i43 

88.858 

188 

116.820 

233 

144.782 

9 

5.592 

54 

33.555 

99 

61  .517 

i44 

89.479 

189 

117.441 

234 

i45.4o3 

10 

6.2l4 

55 

34.176 

IOO 

62.i38 

i45 

90.  100 

190 

n8.o63 

235 

i46.o25 

ii 

6.835 

56 

34-797 

101 

62.760 

i46 

90.  722 

191 

118.684 

236 

146.646 

12 

7.457 

57 

35.419 

1  02 

63.38i 

i4? 

91.343 

192 

ng.3o5 

23? 

147.268 

i3 

8.078 

58 

36.o4o 

io3 

64-  002 

i48 

91  .965 

i93 

119.927 

238 

147.889 

i4 

8.699 

59 

36.662 

io4 

64.624 

i4g 

92.586 

194 

120.548 

23g 

i48.5io 

i5 

9.321 

60 

37.283 

io5 

65.245 

i5o 

93.207 

igS 

121  .170 

24o 

149.1  32 

16 

9.942 

61 

37.904 

1  06 

65.  867 

i5i 

93.829 

196 

121  .791 

241 

i49.753 

'7 

10.563 

62 

38.526 

107 

66.488 

I  52 

94.45o 

197 

122.  4l2 

242 

i5o.375 

18 

n.i85 

63 

3g.i47 

1  08 

67.  109 

i53 

95.071 

198 

123.034 

243 

i5o.996 

19 

1  1.  806 

64 

39.768 

109 

67.731 

1  54 

95.693 

199 

123.655 

244 

i5i  .617 

20 

12.428 

65 

4o.3go 

no 

68.352 

i55 

96.3i4 

200 

124.276 

245 

i52.23g 

21 

i  3.o49 

66 

4i  .on 

in 

68.973 

i56 

96.936 

201 

124.898 

246 

I  52.  860 

22 

13.670 

67 

4i.633 

112 

69.595 

i57 

97.557 

2O2 

125.  5i9 

24? 

i53.48i 

23 

14.292 

68 

42.254 

ii3 

70.216 

i58 

98.178 

203 

126.  i4i 

248 

i54.io3 

24 

i4,gi3 

69 

42.875 

n4 

70.838 

i5g 

98.800 

204 

126.762 

249 

154.724 

25 

i5.535 

70 

43.497 

ii5 

71.459 

160 

99.421 

205 

127.383 

25o 

155.346 

26 

i6.i56 

7i 

44.H8 

116 

72.080 

161 

ioo.o43 

2O6 

128.  oo5 

25l 

i55.g67 

27 

16.777 

72 

44.74o 

117 

72.702 

162 

i  00.664 

207 

128.626 

252 

i56.588 

2S 

17.399 

73 

45.36i 

118 

73.323 

i63 

101.285 

208 

129.248 

253 

157.210 

29 

18.020 

74 

45.982 

119 

73.944 

1  64 

101  .907 

209 

129.869 

254 

iS?.  83i 

3o 

i8.64i 

75 

46.6o4 

I2O 

74.566 

i65 

102.528 

210 

i3o.4go 

255 

i58.452 

3i 

19.263 

76 

47-225 

121 

75.187 

1  66 

io3.  i4g 

211 

l3l  .  112 

256 

i5g.o74 

32 

19.884 

77 

4?.846 

122 

75.809 

167 

103.771 

212 

i3i.733 

257 

i5g.6g5 

33 

2o.5o6 

78 

48.468 

123 

76.430 

168 

104.392 

2l3 

i32.354 

Proportional 

34 

21.127 

79 

49.089 

124 

77.o5i 

169 

io5.oi4 

2l4 

132.976 

Parts. 

35 

21.748 

80 

49.711 

125 

77.673 

170 

io5.635 

2l5 

i33.5g7 

Kil. 

Miles. 

36 

22.370 

81 

5o.332 

126 

78.294 

171 

io6.256 

216 

134.219 

O.  I 

0.062 

37 

22.991 

82 

5o.g53 

127 

78.916 

172 

106.878 

217 

i34.84o 

O.2 

0.  124 

38 

23.6i3 

83 

5i.575 

128 

79.537 

i73 

107.499 

218 

i35.46i 

o.3 

0.186 

39 

24.234 

84 

52.  196 

129 

8o.i58 

i?4 

108.  121 

2I9 

i36.o83 

0.4 

0.249 

4o 

24.855 

85 

52.8i8 

i3o 

80.780 

i75 

108.742 

22O 

i  36.  704 

o.5 

o.3n 

4i 

25.477 

86 

53.439 

i3i 

8i.4oi 

176 

109-363 

221 

i37.326 

0.6 

0.373 

42 

26.098 

87 

54.o6o 

132 

82.022 

177 

109.985 

222 

137.947 

0.7 

0.435 

43 

26.719 

88 

54.682 

i33 

82.644 

178 

110.606 

223 

138.568 

0.8 

0.497 

44 

27.341 

89 

55.3o3 

1  34 

83.265 

179 

in  .227 

224 

i3g.  190 

0.9 

o.55g 

45 

27.962 

9° 

55.924 

i35 

83.887 

180 

in  .849 

225 

i3g.8ii 

I  .0 

0.621 

One  kilometre  equals  0.6213824  English  mile. 


254    TABLE  IV. — FRENCH  FEET  CONVERTED  INTO  ENGLISH  FEET. 


TABLE   IV. 

TO  CONVERT  FRENCH  FEET   INTO  ENGLISH  FEET. 


Fr. 

English. 

Fr. 

English. 

Fr. 

English. 

Fr. 

English. 

Fr. 

English. 

Fr. 

English. 

I 

I  .066 

46 

49.026 

91 

96.986 

136 

I44.944 

1»I 

192  .(;<>:) 

226 

24o.863 

2 

2.  l32 

47 

60.091 

92 

98.060 

i37 

i46.oio 

182 

193.969 

227 

241  .929) 

3 

3.197 

48 

5i  .  167 

93 

99.  116 

i38 

147.076 

i83 

195.035 

228 

242.996  1 

4 

4.263 

49 

52.222 

94 

100.  182 

i3g 

i48.i4i 

1  84 

196.  ioi 

229 

244.o6oi 

5 

5.329 

5o 

53.288 

96 

101  .248 

i4o 

149.207 

186 

197.  167 

23o 

245.  126 

6 

6.3o5 

5i 

54.354 

96 

io2.3i4 

i4i 

160.273 

186 

I98.  232 

23l 

246.  192 

7  7.460 

52 

55.420 

97 

103.379 

i4- 

161.339 

187 

199.298 

232 

247.268 

8  8.626 

53 

56.486 

98 

io4.445 

i43 

162.404 

1  88 

200.  364 

233 

248.323 

9 

9.692 

54 

57.55i 

99 

io5.5i  i 

1  44 

163.470 

189 

201  .43o 

234 

249.389 

10 

10.658 

55 

68.617 

IOO 

106  .  676 

i45 

154.536 

190 

202.496 

235 

25o.455 

ii 

ii  .723 

56 

69.  683 

101 

107.642 

i46 

166.602 

191 

2o3.56i 

236 

261  .621 

12 

12.789 

57 

60.749 

1  02 

108.  708 

i4? 

166.667 

192 

204.627 

237 

262.686 

i3 

i3.855 

58 

6i.8i4 

io3 

109.774 

1  48 

167.733 

193 

206.693 

238 

263.662 

i4 

14.921 

59 

62.880 

io4 

iio.84o 

i4g 

168.799 

i94 

206.768 

239 

264.718 

i5 

15.986 

60 

63.946 

106 

in  .906 

160 

169.866 

i95 

207.824 

24o 

266.784 

16 

17  .062 

61 

66.012 

106 

1  12.971 

161 

160.931 

196 

208.890 

241 

266.849 

i? 

18.118 

62 

66.077 

107 

ii4-o37 

162 

161  .996 

197 

209.966 

242 

267.916 

18 

19.184 

63 

67.143 

1  08 

n5.io3 

i53 

163.062 

198 

211  .O2I 

243 

268.981 

19 

20.260 

64 

68.209 

109 

116.168 

1  54 

164.128 

199 

212.087 

244 

260.047 

20 

21  .3i5 

65 

69.276 

no 

117.234 

i55 

166.  194 

200 

2i3.i53 

245 

261  .  i  i  3 

21 

22.38l 

66 

70.340 

III 

n8.3oo 

166 

166.269 

2OI 

214.219 

246 

262.  178 

22 

23.44? 

67 

71.407 

112 

ng.366 

167 

167.326 

202 

216.286 

247 

263.244 

23 

24.5i3 

68 

72.472 

ii3 

I2O.43I 

168 

i68.39i 

203 

216.  35o 

248 

264.310 

24 

25.578 

69 

73.538 

u4 

121  .497 

169 

169.467 

204 

217.416 

249 

266.376 

25 

26.644 

70 

74.6o4 

u5 

122.563 

160 

170.622 

206 

218.482 

260 

266.441 

26 

27.710 

7i 

76.669 

116 

123.629 

161 

171.688 

206 

219.648 

261 

267.607 

27 

28.776 

72 

76.735 

117 

124.696 

162 

172.664 

207 

220.  6i3 

262 

268.673 

28 

29.841 

73 

77  .801 

118 

126.760 

i63 

173.720 

208 

221  .679 

253 

269.639 

29 

30.907 

74 

78.867 

119 

126.826 

1  64 

174.786 

209 

222.745 

264 

270.  704^ 

3o 

31.973 

76 

79.932 

I2O 

127.892 

166 

176.861 

2IO 

223.811 

266 

271  .770 

3i 

33.o39 

76 

80.998 

121 

128.968 

166 

176.917 

211 

224.877 

266 

272.836 

32 

34.io4 

77 

82.064 

122 

i3o.o23 

167 

177.  983 

212 

226  .942 

267 

273.902 

33 

35.170 

78 

83.i3o 

123 

iSi.oSg 

168 

179.049 

2l3 

227.008 

Proportional 

34 

36.236 

79 

84.196 

124 

i32.i55 

169 

180.  i  i4 

2l4 

228.074 

Pnrts. 

35 

37.302 

80 

86.261 

125 

133.221 

170 

181.180 

216 

229.  i4o 

Jfr. 

English. 

36 

38.368 

81 

86.327 

126 

i34.286 

171 

182  .246 

216 

230.2O5 

O.  I 

o.  107 

3? 

39.433 

82 

87.393 

127 

135.352 

172 

i83.3i2 

217 

23i  .271 

O.2 

O.2l3 

]  38 

40.499 

83 

88.458 

128 

i36.4i8 

i73 

i84-377 

218 

232.337 

o.3 

O.32O 

39 

4i.565 

84 

89.624 

I29 

i37.484 

i?4 

186.443 

219 

233.  4o3 

o.4 

O.426 

4o 

42.63i 

85 

90.690 

i3o 

i38.549 

176 

186.609 

220 

234-468 

0.5 

0.533 

4i 

43.696 

86 

91.666 

i3i 

139.616 

176 

187.676 

221 

235.534 

0.6 

o.63g 

42 

44.762 

87 

92  .  722 

l32 

i4o.68i 

177 

i88.64o 

222 

236.  600 

0.7 

0.746 

43 

45.828 

88 

93.787 

i33 

141.747 

178 

189.  706 

223 

237.666 

0.8 

0.853 

44 

46.894 

89 

94.853 

1  34 

i42.8i3 

179 

190.772 

224 

238-73i 

0.9 

0.969 

45 

47.969 

90 

96.919 

i35 

143.878 

1  80 

191.838 

225 

239.797 

I  .0 

i  .066 

One  French  foot  equals  1.066765  English  foot. 


TABLE  V. — CENTESIMAL  AND  FAHRENHEIT  THEBMOMETEKS.    255 


TABLE   V. 

TO   COMPARE   THE   CENTESIMAL  THERMOMETER  WITH  FAHRENHEIT'S. 


Centes. 

Fahren. 

Centes. 

Fahren. 

Centes. 

Fahren. 

Centes. 

Fahren. 

Proportional  Parts. 

0 

o 

O 

o 

o 

O 

O 

O 

Centes.    Fahren. 

100 

212.  O 

5o 

122.  O 

25 

77.0 

O 

+  32.0 

0                 0 

99 

210.2 

4g.5 

121  .  I 

24.5 

76.  I 

—   I 

30.2 

o.i       o.  18 

98 

208.4 

49 

I2O.2 

24 

75.2 

—    2 

28.4 

0.2      o.36 

97 

206.6 

48.5 

II9.3 

23.5 

74.3 

3 

26.6 

o.3      o.54 

96 

204.8 

48 

II8.4 

23 

73.4 

-  4 

24.8 

0.4      0.72 

95 

2O3.O 

47-5 

II7.5 

22.5 

72.5 

—  5 

23.  O 

o.5      0.90 

94 

2OI  .2 

47 

116.6 

22 

71  .6 

—  6 

21  .2 

0.6      i.  08 

93 

199-4 

46.5 

1  15.7 

21.5 

70.7 

—   7 

I9.4 

0.7      i  .26 

92 

197.6 

46 

114.8 

21 

69.8 

—  8 

17.6 

0.8      i.44 

91 

195.8 

45.5 

113.9 

20.  5 

68.9 

—  9 

i5.8 

0.9      1.62 

90 

O 

194.0 

45 

/  /      C 

n3.o 

20 

f 

68.0 

/?  _      _ 

—  10 

i4-o 

i.o      i.  80 

89 

192.  2 

44.5 

I  12.  I 

19.5 

07  .  i 

—  1  1 

12.2 

88 

190.4 

44 

ni  .2 

J9 

66.2 

—  12 

10.4 

87 

188.6 

43.5 

HO.  3 

i8.5 

65.3 

—  13 

8.6 

86 

186.8 

43 

109.4 

18 

64-4 

-i4 

6.8 

85 

i85.o 

42.5 

io8.5 

i7.5 

63.5 

—  15 

5.o 

84 

i83.2 

42 

107.6 

i? 

62.6 

—  16 

3.2 

83 

181.4 

4i.5 

106.7 

i6.5 

61  .7 

—  17 

+   i.4 

82 

179.6 

4i 

io5.8 

16 

60.8 

—  18 

-  0.4 

81 

177.8 

4o.5 

104.9 

i5.5 

59.9 

—  19 

—    2.2 

80 

176.0 

4o 

io4.o 

i5 

5g.o 

—  20 

-  4.0 

79 

174.2 

39.5 

io3.i 

i4.5 

58.1 

—  21 

—  5.8 

78 

172.4 

39 

I  O2.  2 

i4 

57.2 

—  22 

-  7.6 

w 

0 

77 

170.6 

38.5 

101.3 

:3.5 

56.3 

—  23 

-  9-4 

Q 

1    76 

168.8 

38 

100.4 

i3 

55.4 

-24 

—  II  .2 

(S> 

75 

167.0 

37.5 

99.5 

12.5 

54.5 

—  25 

—  i3.o 

r"f- 

n 

CO 

74 

i65.2 

37 

98.6 

12 

53.6 

—  26 

—  14.8 

B 

73 

i63.4 

36.5 

97-7 

ii.  5 

52.7 

—  27 

—  16.6 

P 

72 

161.6 

36 

96.8 

II 

5i.8 

—  28 

—  18.4 

If 

71 

i5g.8 

35.5 

95.9 

io.5 

5o.9 

—  29 

—  2O.  2 

CO 

70 

i58.o 

35 

95.o 

10 

5o.o 

—  3o 

—  22.0 

o 

69 

i56.2 

34.5 

94.i 

9.5 

49.i 

—  3i 

—  23.8 

+ 

68 

1  54.4 

34 

93.2 

9 

48.2 

—  32 

—25.6 

WhO 

67 

i5a.6 

33.5 

92.3 

8.5 

47-3 

—33 

—27.4 

3 

66 

160.8 

33 

91.4 

8 

46.4 

-34 

—  29.2 

31 

• 

65 

i49.o 

32.5 

9o.5 

7.5 

45.5 

—35 

—  3i.o 

I 

64 

147.2 

32 

89.6 

7 

44.6 

—36 

—32.8 

i 

63 

i45.4 

St.  5 

88.7 

6.5 

43.7 

-37 

—34.6 

B 

CD 

62 

i43.6 

3i 

87.8 

6 

42.8 

—  38 

—36.4 

1* 

61 

i4i.8 

3o.5 

86.  9 

5.5 

41.9 

-39 

—38.2 

60 

i4o.o 

3o 

86.0 

5 

4i  .0 

-4o 

—  4o.o 

59 

1  38.  2 

29.5 

85.i 

4.5 

4o.  i 

-4  1 

—  4i.8 

58 

i36.4 

29 

84.2 

4 

39.2 

-42 

—43.6 

57 

134.6 

28.5 

83.3 

3.5 

38.3 

-43 

—45.4. 

56 

i32.8 

28 

82.4 

3 

37.4 

-44 

—47.2 

• 

55 

i3i.o 

27.5 

8i.5 

2.5 

36.5 

-45 

—49.0 

54 

129.2 

27 

80.6 

2 

35.6 

-46 

—  5o.8 

53 

127.4 

26.5 

79-7 

i.5 

34-7 

-47 

—52.6 

52 

125.6 

26 

78.8 

i 

33.8 

-48 

-54.4 

5i 

123.8 

25.5 

77-9 

o.5 

32  .9 

-49 

—56.2 

256    TABLE  VI. — REAUMUR  AND   FAHRENHEIT  THERMOMETERS 


TABLE   VI. 

TO    COMPARE  REAtnrtJB'S  THERMOMETER  WITH  FAHRENHEIT'S. 


Reaum. 

Fahrenheit 

Reaum. 

Fahrenheit 

Reaum. 

Fahrenheit 

Reaum. 

Fahrenheit. 

o 

O 

0 

O 

o 

0 

O 

O 

80 

212.  0 

4o 

1  22.OO 

2O 

77.00 

O 

+  32.  0 

79 

209.75 

39 

.5 

120.87 

I 

?.5 

75.87 

—    I 

+  29.75 

78 

2O7.5 

39 

119.75 

19 

74.75 

—    2 

+  27.5 

77 

205.25 

38 

.5 

118.62 

i8.5 

73.62 

—   3 

+  25.25 

76 

2O3.O 

38 

II7.5O 

18 

72.  5o 

-  4 

+  23.0 

75 

2OO.75 

3? 

.5 

II6.37 

i7.5 

71.37 

—  5 

+20.75 

74 

igS.S 

37 

Ii5.a5 

17 

70.25 

—  6 

+18.5 

73 

196.25 

36 

.5 

114.12 

i6.5 

69.  12 

—  7 

+  I6.25 

72 

194.0 

36 

i  i  3  .  oo 

16 

68.00 

—  8 

+  i4-o 

71 

191.75 

35 

.5 

in  .87 

i5.5 

66.87 

—  9 

+  11.  75 

70 

iSg.S 

35 

110.75 

i5 

65.  75 

—  10 

+  9.5 

69 

187.25 

34 

.5 

109.62 

i4.5 

64.62 

—  ii 

+     7.25 

68 

i85.o 

34 

108.  5o 

i4 

63.  5o 

—  12 

+  5.o 

67 

182.75 

33 

.5 

107.37 

i3.5 

62.  37 

—  13 

+    2.75 

66 

i8o.5 

33 

io6.25 

i3 

61  .25 

—  14 

+  o.5 

65 

178.25 

32 

.5 

IO5.  12 

12.5 

60.12 

—  15 

-  i.75 

64 

176.0 

32 

104.00 

12 

Sg.oo 

—  16 

-  4.o 

63 

i73.75 

3i 

.5 

102.87 

ii.  5 

57.87 

—  17 

—  6.25 

62 

171.5 

3i 

101.75 

II 

56.  75 

—  18 

£ 

.5 

61 

169.25 

3o 

.5 

100.62 

io.5 

55.62 

—  19 

—  io.75 

60 

167.0  . 

3o 

99.50 

10 

54.  5o 

—  20 

—  i3.o 

59 

164.75 

29 

.5 

98.37 

9.5 

53.  37 

—  21 

—  i5.25 

58 

162.5 

29 

97.25 

9 

52.25 

—  22 

-I7.5 

57 

i6o.25 

28 

.5 

96.  12 

1 

3.5 

5l  .12 

—  23 

—  19 

.75 

56 

i58.o 

28 

gS.O 

» 

3 

So.oo 

—24 

—  22.  O 

55 

i55.75 

27 

.5 

93.8 

7 

7.5 

48.  87 

—  25 

—  24.25 

54 

i53.5 

27 

92.  75 

7 

47-75 

—26 

—  26.5 

53 

1  5  1   25 

26 

.5 

91  .62 

6.5 

46.62 

—27 

—  28.  75 

52 

i49.o 

26 

90.  5o 

6 

45.  5o 

—28 

—  3i.o 

5i 

I46.-75 

25 

.5 

89.37 

5.5 

44.  37 

—29 

—  33.25 

5o 

144.5 

25 

88.25 

5 

43.25 

—  3o 

—35.5 

49 

l42.  25 

24 

.5 

87.  12 

4.5 

42.  12 

—  3i 

-37.75 

48 

i4o.o 

24 

86.00 

4 

4  1  .00 

—  32 

—  4o.o 

4? 

i37.75 

23 

.5 

84.87 

3.5 

39.87 

—33 

—  42.25 

46 

i35.5 

23 

83.  75 

3 

38.  75 

-34 

-44.5 

45 

i33.25 

22 

.5 

82.62 

2.5 

37.62 

—35 

-46.  75 

44 

iSi.o 

22 

8i.5o 

2 

36.  5o 

—36 

—49.0 

43 

128.  75 

21 

.5 

80.  37 

.5 

35.  37 

-37 

—  5i 

.25 

42 

126.5 

21 

•79.25 

I 

34.25 

—38 

—  53.  5o 

4i 

124.25 

2O 

.5 

78.12 

o.5 

33.12 

-39 

—  55.  75 

4o 

122.0 

20 

77.oo 

0 

32.00 

—  4o 

—58.0 

Proportional  Parts. 

0 

0            O 

o 

o 

o 

0             0 

0 

O 

Reaumur  c 

.  I 

O.2      O.3 

0.4 

o.5 

0.6 

0.7    0.8 

0.9 

I  .O 

Fahrenheit  .  .  c 

.22 

0.45  0.67 

0.90 

i  .1 

2  i.35 

i  .67  i  .80 

2.  O2 

2.25 

x°  Reaumur=(32°  j     x°)  Fahrenheit. 


TABLE  VII.— HEIGHT  OF  A  COLUMN  OF  AIR,  ETC.        257 


TABLE   VII. 

HEIGHT    OF   A  COLUMN    OF  AIH    CORRESPONDING    TO   A  TENTH    OF  AN   INCH 
IN   THE   BAROMETER. 


Barom. 

40° 

45° 

50° 

55° 

60° 

65° 

70° 

75° 

80° 

85° 

90° 

Inches. 

Feet. 

Feet. 

Feet 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

22.  0 

121  .5 

122.8 

124.2 

125.5 

126.8 

128.2 

129.5 

i3o.8 

1  32  .  1 

133.5 

134.8 

.2 

I2O.4 

121  .7 

123.  I 

124.4 

125.7 

127.0 

128.3 

129.6 

iSo.g 

l32.2 

133.6 

•  4 

119.3 

120.6 

121  .9 

123.2 

124.6 

125-9 

127.2 

128.5 

129.8 

i3i.i 

i3a.4 

.6 

118.2 

Iig.5 

120.8 

122.  I 

123.4 

124.7 

126.0 

127.3 

128.6 

129.9 

l3l  .2 

.8 

117.2 

II8.5 

II9.8 

121  .1 

122.3 

123.6 

124.9 

126.2 

127.5 

128.8 

i3o.o 

23.0 

116.2 

II7.5 

Il8.7 

I2O.O 

121  .3 

122  .6 

123.8 

125.  I 

126.4 

127.6 

129.9 

.2 

Il5.2 

n6.5 

II7.7 

Iig.O 

I2O.2 

121  .5 

122.  7 

124.0 

125.3 

126.5 

127.  b 

.4 

Il4>2 

u5.5 

116.7 

IlS.O 

II9.2 

I2O.5 

121  .  7 

123.0 

124.2 

125.4 

126.7 

.6 

II3.2 

n4.4 

n5.7 

116.9 

II8.I 

119.4 

120.6 

121.  8 

123. 

124.3 

125.5 

.8 

112.  3 

ii3.5 

n4.8 

1  16.0 

II7.2 

118.4 

119.7 

120.9 

122. 

123.3 

124.6 

24.0 

in  .4 

112.  6 

ii3.8 

n5.o 

116.2 

117.4 

118.7 

119.9 

121  . 

122.3 

123.5 

.2 

no.  5 

111.7 

112.9 

114.  1 

n5.3 

n6.5 

117.7 

118.9 

I2O. 

121.  3 

122.5 

.4 

109.5 

110.7 

111.9 

1  13.  i 

ii4.3 

ii5.5 

116.7 

117.9 

II9. 

I2O.3 

121  .5 

.6 

108.6 

109.8 

in  .0 

I  12.2 

u3.4 

n4.6 

ii5.8 

116.9 

118. 

119.3 

120.5 

.8 

107.8 

108.9 

no.  i 

in.  3 

I  12  .5 

n3.7 

n4.8 

1  16.0 

117.2 

118.4 

Iig.S 

25.  O 

106.9 

108.1 

109.2 

110.4 

ii  i  .6 

112.7 

113.9 

ii5.  i 

116.2 

117.4 

118.6 

.2 

106.0 

107.2 

108.4 

109.5 

110.7 

in.  8 

iiS.o 

ii4.i 

ii5.3 

ii6.5 

117.6 

.4 

IO5.2 

io6.4 

107.5 

108.7 

109.8 

I  I  I  .0 

112.  I 

ii3.3 

ii4.4 

ii5.6 

116.7 

.6 

104.4 

io5.5 

106.7 

107.8 

108.9 

I  10.  I 

III  .2 

112.  4 

ii3.5 

ii4.6 

iiS.fc 

.8 

io3.6 

104.7 

io5.8 

107.0 

108.1 

109.2 

no.  4 

i  ii  .5 

112.  6 

n3.8 

114.9 

26.0 

IO2.8 

103.9 

io5.o 

1  06.  1 

107.3 

108.4 

109.6 

1  10.6 

in.  8 

112.9 

ii4.o 

.2 

IO2.0 

io3.i 

104.2 

io5.3 

1  06.  5 

107.6 

108.7 

109.8 

110.9 

112.  O 

iiS.i 

.4 

IOI  .2 

102.3 

io3.4 

io4-6 

105.7 

106.8 

107.9 

109.0 

110  .1 

II  I  .2 

112.  3 

.6 

IOO.5 

101  .6 

102.7 

io3.8 

104.9 

1  06  .0 

107.  i 

108.2 

109.3 

II0.4 

in  .4 

.8 

99-7 

100.8 

IOI  .9 

io3.o 

104.  i 

io5  .2 

io6.3 

107.4 

io8.5 

lOg.S 

110.6 

27.0 

99.0 

too.  i 

IOI  .2 

I  O2.  2 

io3.3 

io4.4 

io5.5 

1  06.  6 

107.6 

108.7 

109.8 

.2 

98.3 

99.3 

I  OO.4 

101  .5 

102  ,6 

io3.6 

104.7 

io5.8 

106.8 

107.9 

109.0 

.4 

97.5 

98.6 

99-7 

100.7 

101.  8 

IO2  .9 

103.9 

io5.o 

1  06.  i 

107.  I 

108.2 

.6 

96.8 

97-9 

98.9 

IOO.O 

IOI  .  I 

I  O2  .  I 

103.2 

104.2 

io5.3 

io6.3 

107.4 

.8 

96.  i 

97.2 

98.2 

99-3 

100.3 

IOI  .4 

102.4 

io3.5 

io4.5 

io5.6 

1  06.  6 

28.0 

95.4 

96.5 

97.5 

98.6 

99.6 

100.6 

:oi  .7 

102.7 

io3.8 

io4.8 

105.9 

.2 

94.8 

95.8 

96.8 

97-9 

98.9 

99.9 

IOI  .O 

IO2.0 

io3.o 

104.  i 

io5.i 

.4 

94.1 

95.i 

96.  i 

97.2 

98.2 

99.2 

IOO.2 

ioi.  3 

1  02.  3 

io3.3 

io4.3 

.6 

93.4 

94.4 

95.5 

96.5 

97.5 

98.5 

99.5 

too.  6 

ioi  .6 

102.6 

io3.6 

.8 

92.8 

93.8 

94.8 

95.8 

96.8 

97.8 

98.8 

99.8 

100.8 

ioi.  8 

102.8 

29  .0 

92.1 

93.i 

94.1 

95.  i 

96.2 

97.2 

98.2 

99.2 

IOO.2 

IOI  .2 

102.2 

.2 

91  .5 

92.5 

93.5 

94.5 

95.5 

96.5 

97.5 

98.5 

99.5 

100.5 

ioi  .5 

.4 

90.9 

91.9 

92.9 

93.9 

94-8 

95.8 

96.8 

97.8 

98.8 

99.8 

100.8 

.6 

90.3 

91.3 

92.2 

93.2 

94.2 

95.2 

96.2 

97.2 

98.2 

99.1 

IOO.  I 

.8 

89.7 

90.6 

91  .6 

92.6 

93.6 

94.5 

95.5 

96.5 

97.5 

98.5 

99-4 

3o.o 

89.1 

90.0 

91  .0 

92.0 

92.9 

93.9 

94.9 

95.9 

96.8 

97.8 

98.8 

.2 

88.5 

89.4 

90.4 

91.4 

92.3 

93.3 

94/3 

95.2 

96.2 

97.2 

98.1 

.4 

87.9 

88.8 

89.8 

90.8 

91.7 

92.7 

93.6 

94.6 

95.6 

96.5 

97.5 

.6 

87.3 

88.2 

89.2 

90.2 

91.1 

92.  i 

93.0 

g4.o 

gS.o 

95.9 

96.8 

.8 

86.7 

87.6 

88.6 

89.6 

go.S 

9i.5 

92.4 

93.4 

94.3 

95.2 

96.2 

R 


258    TABLE  VIII. — FOR  REDUCING  BAROMETRIC  OBSERVATIONS 


TABLE  VIII. 

FOR  REDUCING  BAROMETRIC  OBSERVATIONS  TO  THE  FREEZING   POINT. 


Temp. 

27 

27.5 

28 

28.5 

29 

29.5 

30 

30.5 

31   Temp. 

O 

+  .069 

.071 

.072 

.073 

.074 

.076 

.077 

.078 

.080 

0 

I 

.067 

.068 

.069 

.071 

.072 

.073 

.074 

.076 

.077 

I 

2 

.064 

.066 

.067 

.068 

.069 

.070 

.072 

.073 

.074 

2 

3 

.062 

.o63 

.064 

.o65 

.067 

.068 

.069 

.070 

.071 

3 

4 

,o5() 

.061 

.062 

.o63 

.o64 

.o65 

.066 

.067 

.068 

4 

5 

.oS? 

.o58 

.069 

.060 

.061 

.062 

.o63 

.o65 

.066 

5 

6 

+  .o55 

.o56 

.057 

.o58 

.o5g 

.060 

.061 

.062 

.o63 

6 

7 

.052 

.o53 

.o54 

.o55 

.o56 

.057 

.o58 

.oSg 

.060 

7 

8 

.o5o 

.o5i 

.o52 

.o53 

.o54 

.o54 

.o55 

.o56 

.057 

8 

9 

•  o4? 

.o48 

•  o4g 

.o5o 

.o5i 

.052 

.o53 

.o54 

.o54 

9 

10 

.o45 

.o46 

.047 

.047 

.o48 

.049 

.o5o 

.<>•")  i 

.o52 

10 

ii 

+  .042 

.o43 

•  o44 

.o45 

.o46 

.o46 

.047 

.048 

.049 

ii 

12 

.o4o 

.o4i 

.042 

.042 

.o43 

•  o44 

.o45 

.o45 

.o46 

12 

i3 

.o38 

.o38 

.o3g 

.o4o 

.o4o 

.04  1 

.042 

.043 

.o43 

i3 

i4 

.o35 

.o36 

.037 

.037 

.o38 

.o38 

.o3g 

.o4o 

.o4o 

i4 

i5 

.o33 

.o33 

.o34 

.o35 

.o35 

.o36 

.o36 

.037 

.o38 

i5 

16 

+  .o3o 

.o3i 

.032 

.032 

.o33 

.o33 

.o34 

.o34 

.o35 

16 

i? 

.028 

.028 

.029 

.o3o 

.o3o 

.o3i 

.o3i 

.032 

.o32 

17 

18 

.025 

.026 

.026 

.027 

.027 

.028 

.028 

.029 

.029 

18 

'9 

.023 

.024 

.024 

.024 

.025 

.025 

.026 

.026 

.027 

J9 

20 

.021 

.021 

.021 

.022 

.022 

.023 

.023 

.023 

.024 

20 

21 

+  .018 

.019 

.019 

.019 

.020 

.020 

.020 

.021 

.021 

21 

22 

.016 

.016 

.016 

.017 

.017 

.017 

.018 

.018 

.018 

22 

23 

.oi3 

.014 

.014 

.014 

.oi4 

.oi5 

.oi5 

.oi5 

.oi5 

23 

24 

.Oil 

.on 

.on 

.012 

.012 

.012 

.012 

.012 

.oi3 

24 

25 

.009 

.009 

.009 

.009 

.009 

.009 

.009 

.010 

.010 

25 

26 

+  .006 

.006 

.006 

.006 

.007 

.007 

.007 

.007 

.007 

26 

27 

4-.oo4 

.oo4 

.004 

.oo4 

.oo4 

.004 

.004 

.004 

.004 

27 

28 

+  .OOI 

.001 

.001 

.001 

.001 

.001 

.001 

.001 

.001 

28 

29 

—  .001 

.001 

.001 

.001 

.001 

.001 

.001 

.001 

.001 

29 

3o 

—  .oo4 

.oo4 

.oo4 

.oo4 

.oo4 

.oo4 

.004 

.oo4 

.oo4 

3o 

3i 

—  .006 

.006 

.006 

.006 

.007 

.007 

.007 

.007 

.007 

3i 

32 

.008 

.009 

.009 

.009 

.009 

.009 

.009 

.010 

.010 

32 

33 

.on 

.Oil 

.on 

.012 

.012 

.012 

.012 

.012 

.012 

33 

34 

.oi3 

.oi4 

.014 

.014 

.014 

.oi5 

.oi5 

.oi5 

.oi5 

34 

35 

.016 

.016 

.016 

.017 

.017 

.017 

.018 

.018 

.018 

35 

36 

—  .018 

.019 

.019 

.019 

.020 

.020 

.020 

.021 

.021 

36 

37 

.021 

.021 

.021 

.022 

.022 

.022 

.023 

.023 

.024 

3? 

38 

.023 

.023 

.024 

.024 

.025 

.025 

.026 

.026 

.026 

38 

39 

.025 

.026 

.026 

.027 

.027 

.028 

.028 

.029 

.029 

39 

4o 

.028 

.028 

.029 

.029 

.o3o 

.o3o 

.o3i 

.o3i 

.032 

4o 

4i 

—  .o3o 

.o3i 

.o3i 

.032 

.o33 

.o33 

.o34 

.o34 

,o35 

4i 

42 

.o33 

.o33 

.o34 

.o34 

.o35 

.o36 

.o36 

.037 

.037 

42 

43 

.o35 

.o36 

.o36 

.037 

.o38 

.o38 

.oSg 

.o4o 

.o4o 

43 

44 

.087 

.o38 

.039 

.o4o 

.o4o 

.  o4i 

.042 

.042 

.043 

44 

45 

.o4o 

.o4i 

.o4i 

.042 

.o43 

.o44 

•  o44 

.o45 

.o46 

45 

TO   THE   FREEZING   POINT. 


259 


TABLE    VIII. 

FOR  REDUCING  BAROMETRIC   OBSERVATIONS  TO  THE  FREEZING   POINT. 


Temp. 

27 

27-5 

28 

28.5 

29 

29.5 

30 

30.5 

31 

Temp. 

45 

—  .o4o 

.  t>4i 

.o4i 

.042 

.043 

•  o44 

.044 

.o45 

.o46 

45 

46 

.042 

.o43 

.044 

.o45 

.o45 

.o46 

.047 

.o48 

.049 

46 

47 

.o45 

.o46 

.046 

.047 

.o48 

•  o4g 

.o5o 

.o5i 

.o5i 

47 

48 

.047 

.o48 

.049 

.o5o 

.o5i 

.o52 

.052 

.o53 

.o54 

48 

49 

.o5o 

.o5o 

.o5i 

.o52 

.o53 

,o54 

.o55 

.o56 

.057 

49 

5o 

.052 

.o53 

.o54 

.o55 

.o56 

.067 

.o58 

.oSg 

.060 

5o 

5i 

—  .o54 

.o55 

.o56 

.057 

.o58 

.o5g 

,060 

.061 

.062 

5i 

52 

.057 

.o58 

.069 

.060 

.061 

.062 

.o63 

.o64 

.o65 

52 

53 

.oSg 

.060 

.061 

.o63 

.064 

.o65 

.066 

.067 

.068 

53 

54 

.062 

,o63 

.o64 

.o65 

.066 

.067 

.068 

.070 

.071 

54 

55 

.o64 

.o65 

.066 

.068 

.069 

.070 

.071 

=  072 

.073 

55 

56 

—  .066 

.068 

.069 

.070 

.071 

.073 

.074 

.075 

.076 

56 

5? 

.069 

.070 

.071 

.073 

.074 

.075 

.076 

.078 

.079 

57 

58 

.071 

.073 

.074 

.075 

.077 

.078 

.079  .081 

.082 

58 

59 

.074 

.075 

.076 

.078 

.079 

.080 

.082  i  ,o83 

.o85 

59 

Go 

.076 

.077 

.079 

.080 

.082 

.o83 

.o85 

.086 

.087 

60 

61 

—  .078 

.080 

.081 

.o83 

.084 

.086 

.087 

.089 

.090 

61 

62 

.081 

.082 

.o84 

.o85 

.087 

.088 

.090 

.091 

.093 

62 

63 

.o83 

.o85 

.086 

.088 

.089 

.091 

.093 

.094 

.096 

63 

64 

.086 

.087 

.089 

.090 

.092 

.094 

.096 

.097 

.098 

64 

65 

.088 

.090 

.091 

.093 

.og5 

.096 

.098 

.  IOO 

.  IOI 

65 

66 

—  .090 

.092 

.094 

.096 

.097 

.099 

.  IOI 

.  1  02 

.  io4 

66 

67 

.ogS 

.og5 

.096 

.098 

.  IOO 

.  1  02 

.io3 

.  io5 

.  107 

67 

68 

.ogS 

.097 

.099 

.  IOI 

.  1  02 

.  1  1>4 

.  1  06 

.108 

.  109 

68 

69 

.098 

.  IOO 

.  101 

,io3 

.  io5 

.  107 

.109 

.  I  10 

.  112 

69 

70 

.  IOO 

.  102 

.  io4 

.  106 

.108 

.  109 

.in 

.ii3 

.ii5 

70 

71 

—  .  102 

.  io4 

.  1  06 

.108 

.  no 

.  112 

.114 

.116 

.118 

?i 

72 

.  io5 

.  107 

.  109 

.in 

.ii3 

.ii5 

.117 

.119 

.  120 

72 

73 

.107 

.  109 

.III 

.ii3 

.1*5 

.117 

.119 

.  121 

.123 

73 

74 

.  no 

.112 

.114 

.116 

.118 

.  120 

.  122 

.  124 

.  126 

74 

75 

.  112 

.n4 

.116 

.118 

.  I2O 

.  122 

.125 

.  127 

.  129 

?5 

76 

-.114 

.117 

.119 

.  121 

.123 

.  125 

.  127 

.  129 

,i3i 

76 

77 

.117 

.119 

.  121 

.123 

.126 

.128 

.i3o 

.132 

.i34 

77 

78 

.119 

.  122 

.  124 

.126 

.128 

.  i3o 

.i33 

.i35 

.i37 

78 

79 

.  122 

.  124 

.  126 

.128 

.i3i 

.i33 

.i35 

.i37 

.  i4o 

79 

80 

.  124 

.  126 

.  129 

.13! 

.i~33 

.i36 

.1.38 

.  i4o 

.i43 

80 

81 

—  .126 

.  129 

.i3i 

.i34 

.i36 

.138 

.i4i 

.i43 

.i45 

81 

82 

.  129 

,l3l 

.i34 

.136 

.i38 

.i4i 

.i43 

.i46 

.i48 

82 

83 

,i3i 

.134 

.i36 

.189 

.i4i 

.143 

.i46 

.i48 

.i5i 

83 

84 

.i34 

.i36 

.i3g 

.i4i 

•  i44 

.i46 

.149 

.i5i 

.i54 

84 

85 

.186 

.  1  3g 

.141 

•  i44 

.i46 

.149 

.i5i 

.i54 

.i56 

85 

86 

—  .i38 

.i4i 

•  i44 

.146 

.i4g 

.i5i 

.i54 

.i56 

.169 

86 

87 

.i4i 

.i43 

.i46 

.i4g 

.i5i 

.i54 

.i57 

.  i5g 

.  162 

87 

88 

.i43 

.i46 

.i4g 

.i5i 

.i54 

.i57 

.  i5g 

.  162 

.i65 

88 

89 

.i46 

.i48 

.i5i 

.i54 

.i56 

.  i5g 

.  162 

.165 

.  167 

89 

90 

.148 

.i5i 

.i53 

.i56 

.  i5g 

.  162 

.i6A 

.  167 

.  I7O 

9° 

TA.BLE   IX. — ALTITUDES   WITH   THE   BAROMETER. 


TABLE   IX. 

ALTIT0DE8   WITH   THE   BAROMETER. — Part  I. 


Inches. 

Feet. 

Inches. 

Feet 

Inches. 

Feet. 

Inches. 

Feet. 

1  1  .0 

1396.9 

l6.0 

I1I86.3 

21  .O 

18291  .0 

26.0 

23871  .0 

.1 

1633.3 

.  I 

Il349.  I 

.  I 

l84i5.i 

.  I 

23971.3 

.2 

1867.6 

.2 

11610.9 

.2 

18538.7 

.2 

24071  .2 

.3 

2099.9 

.3 

11671  .7 

.3 

18661.6 

.3 

24170.7 

.4 

233o.  i 

.4 

n83i.5 

.4 

18784.0 

.4 

24269.8 

.5 

2558.3 

.5 

11990.3 

.5 

18906.8 

.5 

24368.6 

.6 

2784.5 

.6 

I2I48.2 

.6 

19027.0 

.6 

24467.0 

•7 

3oo8  .  7 

•7 

I23o5.  i 

•7 

19147.7 

•7 

24565.1 

.8 

323i.i 

.8 

12461  .0 

.8 

19267.8 

.8 

24662.7 

•9 

345i.6 

•9 

12616.  1 

•9 

19387.4 

•9 

24760.0 

12.  O 

3670.2 

17.0 

12770.2 

22.  O 

19606.4 

27  .0 

24867.0 

.  I 

3887.0 

.  i 

12923.5 

.  I 

19624.9 

.  i 

24963.6 

.2 

4lO2.O 

.2 

13076.  8 

.2 

19742.9 

.2 

26049.8 

.3 

43x5.3 

.3 

13227.3 

.3 

19860.3 

.3 

26145.7 

•  4 

4526.9 

.4 

i3377.9 

.4 

19977.2 

.4 

2524l  .2 

.5 

4?36.7 

.5 

13627.6 

.5 

20093.6 

.5 

25336.4 

.6 

4g44-9 

.6 

13676.6 

.6 

20209.4 

.6 

25431.2 

•7 

5i5i.4 

•7 

i3824.5 

•7 

20324.8 

•7 

25626.7 

.8 

5356.4 

.8 

13971.7 

.8 

20439.6 

.8 

26619.9 

•9 

5559.7 

•9 

i4i  18.0 

•9 

20664.0 

•9 

26713.7 

1  3.o 

6761.4 

18.0 

14263.6 

23.0 

20667.8 

28.0 

26807.  i 

.  i 

6961  .6 

.  i 

i44o8.3 

.  I 

20781  .  I 

.  i 

26900.3 

.2 

6i6o.3 

.2 

14552.3 

.2 

20894.0 

.2 

26993.  i 

.3 

6357.5 

.3 

14696.4 

.3 

21006.4 

.3 

26086.6 

.4 

6553.2 

.4 

i4837.8 

.4 

21118.3 

.4 

26177.  7 

.5 

6747.5 

.5 

14979.4 

.5 

21229.7 

.5 

26269.6 

.6 

6940.3 

.6 

16120.  3 

.6 

2i34o.6 

.6 

2636i.i 

•7 

7i3i.7 

•7 

16260.3 

•7 

21461  .1 

•7 

26452.3 

.8 

7321.7 

.8 

16399.7 

.8 

21661.  i 

.8 

26543.2 

•9 

75io.3 

•9 

i5538.3 

•9 

21670.6 

•9 

26633.7 

14.0 

7697.6 

19.0 

16676.2 

24.0 

21779.7 

29.0 

26724.0 

.  i 

7883.6 

.  i 

i58i3.3 

.  i 

21888.4 

.  i 

26813.9 

.2 

8068.2 

.2 

16949.8 

.2 

21996.6 

.2 

26903.6 

.3 

825i.5 

.3 

16086.6 

.3 

22104.  3 

.3 

26992  .8 

•  4 

8433.6 

.4 

16220.6 

.4 

2221  I  .6 

.4 

27081  .9 

•  J 

86i4-4 

.5 

i6354.8 

.5 

223l8.4 

.5 

27170.6 

.6 

8794.0 

.6 

16488.5 

.6 

22424.8 

.6 

27269.0 

.7 

8972.3 

•7 

16621  .4 

•  7 

2253o.8 

•7 

27347.1 

.8 

9i49.5 

.8 

16763.7 

.8 

22636.4 

.8 

27434.9 

•9 

9325.5 

•9 

16885.3 

•9 

22741  .5 

•9 

27622.6 

i5.o 

gSoo.S 

20.  o 

17016.3 

26.0 

22846.3 

3o.o 

27609.7 

.  i 

9673.8 

.1 

17146.6 

.  i 

22960.6 

.  i 

27696.6 

.2 

9846.2 

.2 

17276.3 

.2 

23o64-4 

.2 

27783.3 

.3 

10017.5 

.3 

I74o5.3 

.3 

23167.9 

.3 

27869.7 

.4 

10187.7 

•  4 

17633.7 

.4 

23261  .0 

.4 

27966.7 

.5 

io356.8 

.5 

17661  .4 

.5 

23363.6 

.5 

28o4i.,5 

!  .6 

10624.8 

.6 

17788.6 

.6 

23466.9 

.6 

28127  ' 

i  -7 

10691.8 

•7 

17916.1 

•7 

23667.7 

•7 

28212.3 

.8 

10867.7 

.8 

i8o4i  .0 

.8 

2366g.2 

.8 

28297.3 

._li 

IIO22.5 

•9 

18166.  3 

•9 

23770.3 

•9 

28382.0  ] 

TABLE   IX. — ALTITUDES   WITH  THE   BAROMETER. 


261 


TABLE    IX. 

ALTITUDES   "WITH   THE   BAROMETER  —  Part  II. 


T—  1". 

Feet. 

T—  T'. 

Feet. 

T—  T.         Feet. 

T—  T'. 

1  eet. 

T—  '1  '.         i  e,  t. 

1° 

2.3 

17° 

by.  6 

33° 

77.3 

49° 

114.7 

00° 

I  J2.2 

2 

4-7 

18 

42.  I 

34 

79.6 

5o 

117.0 

66 

i54.5 

3 

7.0 

»g 

44.5 

35 

81.9 

5! 

119.4 

67 

i56.8 

4 

9-4 

20 

46.8 

36 

84.3 

52 

121.7 

68 

159.2 

5 

11.7 

21 

49.2 

37 

86.6 

53 

I24.I 

69 

i6i.5 

6 

i4-o 

22 

5i.5 

38 

89.0 

54 

126.4 

7° 

163.9 

7 

16.4 

23 

53.8 

39 

91.3 

55 

128.7 

7i 

166.2 

8 

18.7 

24 

56.2 

4o 

93.6 

56 

i3i.i 

72 

168.6 

9 

21  .  I 

25 

58.5 

4i 

96.0 

57 

i33.4 

73 

170.9 

10 

23.4 

26 

60.9 

42 

98.3 

58 

i35.8 

74 

173.3 

1  1 

25.8 

27 

63.2 

43 

100.7 

59 

i38.i 

?5 

175.6 

12 

28.1 

28 

65.5 

44 

io3.o 

60 

i4o.4 

76 

177.9 

i3 

3o.4 

29 

67.9 

45 

io5.3 

61 

142.8 

77 

i8o.3 

i4 

32.8 

3o 

70.2 

46 

107.7 

62 

i45.i 

78 

182.6 

i5 

35.i 

3i 

72.6 

47 

IIO.O 

63 

i47,5 

79 

i85.o 

16 

37.5 

32 

74-9 

48 

112.  4 

64 

i4g.8 

80 

187.3 

Parts  III.,  IV,  and  V. 


Part  III. 

V 

f 

Part  V. 

s 

h 

•a  3 

Positive  from  Lat.  o°  to  45°. 
Negative  from  Lat.  45°  to  90°. 

*1 

Always  positive. 

SI 

Latitude. 

*R 

Height  of  Barometer  at  Lower  Station. 

o.< 
< 

O° 

10° 

20° 

3o° 

4o° 

/RO 

*f 

C 

a 

a 

d 

C 

d 

90 

80 

70 

60 

5o 

40 

1  •* 

00 

o 

N 

O) 
C! 

»«$ 

«o 

0) 

oo 

N 

Feec. 

Feet. 

Feet 

Feet. 

Feet. 

Feet. 

Feet.!  Feet. 

Feet. 

Feet. 

Fiet. 

teet. 

1-eet 

Feet. 

I,OOO 

2.6 

2.5 

2.0 

1.3 

0.5 

O 

2.5 

1.3 

I  .0 

0.8 

0.6 

0.4 

0.  2 

2,000 

5.3 

5.0 

4-1 

2.6 

0.9 

O 

5.2 

2.5 

2.O 

i.5 

I  .  I 

0.7 

0.3 

3,OOO 

7-9 

7.5 

6.1 

4.0 

1.4 

O 

7-9 

3.8 

3.o 

2.3 

i-7 

1  .1 

0.5 

4,OOO 

10.6 

IO.O 

8.1 

5.3 

1.8 

O 

10.8 

5.i 

4.0 

3.i 

2.2 

1.4 

0.7 

5,OOO 

13.2 

12.4 

IO.  I 

6.6 

2.3 

O 

i3.7 

6.4 

5.o 

3.8 

2.8 

1.8 

0.8 

6,OOO 

iS.g 

14.9 

12.2 

7-9 

2.» 

O 

16.7 

7.6 

6.0 

4.6 

3.3 

2.  I 

I  .0 

7,OOO 

i8.5 

17.4 

14-  - 

9.3 

3.2 

0 

19.9 

8.9 

7-i 

5.4 

3.9 

2.5 

I  .2 

8,OOO 

21  .2 

19.9 

16.2 

10.6 

3.7 

O 

23.! 

IO.2 

8.j 

6.2 

4.4 

2.8 

i.3 

9.OOO 

23.8 

22.4 

i8.3 

n.  t) 

4.i 

0 

26.4 

ii.  4 

9.1 

6.9 

b.o 

3.2 

1.5 

IO,OOO 

26.5 

24.9 

20.  3 

13.2 

4.6 

O 

29.8 

12.7 

IO.  I 

7-7 

5.5 

3.5 

i-7 

I  I,OOO 

29.  1 

27.4 

22.  6 

i4.6 

5.i 

O 

33.3 

i4-o 

ii  .1 

8.5 

6.1 

3.9 

1.8 

I2,OOO 

3i.8 

29.9 

24.4 

15.9 

5.5 

O 

36.9 

i5.3 

12.  I 

9.2 

6.6 

4.2 

2.0 

I  3,OOO 

34.4 

32.4 

26.4 

17.2 

6.0 

O 

4o.6 

i6.5 

i3.i 

IO.O 

7.2 

4.6 

2.2 

i4,ooo 

37.! 

34-9 

28.4 

i8.5 

6.4 

O 

44-4 

17.8 

14.1 

10.8 

7-7 

4.9 

2.3 

i5,  000(39.7 

37.3 

3o.4 

19.9 

6.9 

O 

48.3 

19.1 

i5.| 

ii.  5 

8.3|  5.3 

2.5 

16,000 

42.4 

39.8 

32.5 

21.2 

7-4 

O 

52.3 

20.3 

16.1 

12.3 

8.8 

5.6 

2.7 

17,000 

45.o 

42.3 

34.5 

22.5 

7.8 

O 

56.4 

21.6 

17.1 

i3.i 

9-4 

6.0 

2.8 

18,000 

47-7 

44-  8 

36.5 

23.8 

8.3 

O 

6o.5 

22.9 

18.1 

i3.8 

9.9 

6.3 

3.o 

19,000 

5o.3 

47.3 

38.6 

25  .2 

8.7 

O 

64.8 

24.1 

19.2 

i4.6 

io.5 

6.7 

3.2 

20,000 

53.o 

49.8 

4o.6 

26.5 

9.2 

O 

69.2 

25.4 

2O.  2 

i5.4 

II  .0 

7.0 

3.3 

21,000 

55.6 

52.3 

42.6 

27.8 

9-7 

0 

73.6 

26.7 

21.2 

16.1 

i  i  .6  7.4 

3.5 

22,000 

58.3 

54.8 

44-7 

29  .  I 

10.  I 

O 

78.2 

28.0 

22.2 

16.9 

12.  I     7.7 

3.7 

23,OOO 

60.9 

57.3 

46.  7 

3o.5 

10.6 

O 

82.9 

29.2 

23.2 

17.7 

12.7;  8.  i 

3.8 

24,000 

63.6 

59.8 

48.7 

3i.8 

1  1  .0 

0 

87.6 

3o.5 

24.2 

i8.5 

i3.2  8.4 

4.o 

25.OOO 

66.2 

62.2 

5o.  7 

33.i 

ii.  5 

O 

92.5 

3i.8 

25.2 

19.2 

i3.8  8.8 

4.i 

262      TABLE   X. — MEAN   HEIGHT   OF  THE   BAROMETEK,  ETC. 


TABLE   X. 

MEAN   HEIGHT  OF  THE  BAROMETER  IN   THE  DIFFERENT  MONTHS. 


George- 
town. 

Havana. 

Natchez. 

St.  Louis. 

I'hilmlel- 
phia. 

Boston. 

To™*-    Bo™. 

Kenn. 
Harbor. 

Jan.  .  . 

29.  942 

3o.  129 

29.779 

29.602 

29.961 

29.976 

29.618:29.716 

29.773 

Feb.  .  . 

.965 

29.928 

.733 

.586 

.908 

•957 

.6i4 

.886 

.843 

March 

•957 

.960 

.723 

.559 

.942 

.886 

.622 

3o.  107 

•  745 

April. 

.945 

.goS 

.664 

.490 

.924 

.897 

.657 

.068 

.898 

May.. 

.933 

.85l 

.623 

.465 

.886 

.907 

.565 

.o5i 

•937 

June  . 

.962 

•  948 

.608 

•  4?8 

.891 

.878 

.577 

29.888 

.714 

July  .  . 

.971 

•  948 

.646 

.523 

•  9i5 

•  936 

.589 

.817 

.736 

Aug.  . 

.964 

.816 

.624 

.542 

•  943 

.965 

.638 

.683 

.689 

Sept.  . 

•  943 

.821 

•749 

.56i 

.971 

•994 

.64? 

.689 

.653 

Oct.  .  . 

.914 

,85i 

.722 

.588 

.949 

.964 

.663 

.784 

•  75o 

Nov... 

.885 

.971 

.765 

.588 

.941 

.955 

.626 

.899 

.753 

Dec.  .  . 

.910 

3o.o65 

.816 

.601 

•959 

.953 

.643 

.869 

•  748 

Year 

29.939 

29.933 

29.696 

29.548 

29.932 

29.939 

29.621129.886 

29.765 

(Jhris- 

Pai-A 

Constan- 

p  • 

Green- 

St. Pe- 

Arch- 

iiiim- 

tianborg. 

Aden. 

v^airu. 

tinople. 

ang. 

wich. 

tersburg. 

angel. 

merfeat. 

Jan.  .  . 

29.829 

29.823 

3o.oi6 

Jo.OOg 

29.877 

29.760 

3O.O22 

29.728 

29.605 

Feb... 

.840 

.844 

.016 

29.982 

.886 

.804 

•  o44 

•  743 

.456 

March 

•  874 

.776 

29.899 

3o.oi4 

•777 

.769 

29.961 

.698 

•  7°4 

April. 

•939 

.701 

.926 

29.833 

.732 

.760 

.969 

.767 

.816 

May.. 

.971 

.610 

.852 

.899 

•749 

.771 

.968 

•774 

.888 

June  . 

.968 

.528 

.702 

.852 

.8i4 

.787 

.  V>  '•' 

.690 

.818 

July  .  . 

.920 

.482 

.682 

.845 

.785 

.801 

.853 

.677 

.816 

Aug.  . 

.882 

.512 

.688 

.860 

•794 

.789 

.919 

.672 

.771 

Sept.  . 

.86a 

.632 

.792 

.961 

.788 

.819 

.969 

.763 

•  74o 

Oct.  .  . 

.849 

.778 

.910 

3o.o33 

.702 

.696 

.954 

.7i3 

.658 

Nov... 

.862 

.876 

.961 

.  io4 

.754 

.754 

.845 

.680 

.684 

Dec.  .  . 

.838 

.890 

.994 

.oi4 

.729 

.821 

.g3i 

.675 

.568 

Year 

29.886 

29.704  29.866 

29.943 

29.782 

29.777 

29.944 

29.711129.710 

Singa- 
pore. 

Madras. 

Bombay. 

Canton. 

Benares. 

Pekin. 

Tiflis. 

Nerts- 
chinsk. 

Jakntsk. 

Jan.  .  . 

29.917 

29.999 

29.941 

3o.  176 

29.741 

30.498 

28.64o 

27.989 

29   896 

Feb.  .  . 

.916 

.970 

.925 

.099 

.642 

.429 

.627 

.960 

.868 

March 

.884 

.890 

.872 

.018 

.575 

.254 

.534 

.874 

•749 

April. 

.886 

.828 

•199 

29.849 

.4a3 

.091 

•477 

•  744 

.620 

May.. 

.872 

.709 

.762 

.761 

.33i 

29.940 

.468 

•597 

.472 

June  . 

.858 

.688 

•  649 

.780 

.179 

.788 

•4zi 

.690 

.366 

July  .  . 

.868 

.711 

.656 

.656 

.161 

•  746 

.892 

.56i 

.383 

Aug.  . 

.880 

•  747 

.722 

.659 

.265 

.860 

•  446 

.65o 

.435 

Sept.  . 

.886 

.783 

.791 

.696 

.870 

30.079 

.555 

.768 

.711 

Oct.  .  . 

.898 

.863 

.833 

.911 

.543 

.268 

.662 

.858 

.670 

Nov... 

.866 

.943 

.895 

36.071 

.649 

•  4io 

.689 

.898 

.828 

Dec.  .  . 

.884 

.964 

•  944 

.123 

•  744 

.482 

.628 

.901 

3o.o6o 

Year 

29.884 

29.842 

29.815 

29.895 

29.468 

3o.i54 

28.538 

27.782 

20.679 

TABLES  XI,  XII. — MEAN  HEIGHT   OF  THE   BAROMETER,  ETC.    263 


TABLE   XL 

MEAN  HEIGHT   OP  THE  BAROMETER  FOR  ALL  HOURS   OF  THE  DAT. 


Equator. 

Cumana. 

Calcutta. 

Philadel- 
phia. 

Toronto. 

Padua. 

Green- 
wich. 

St.  Pe- 
tersburg. 

\an    " 
Renns'r. 

Midnight 
1A.M. 
2 
3 
4 
5 

29.626 
.6l5 
.698 
.592 

.58o 
.593 

29.798 
.785 
.772 
.760 

,75i 

.756 

29.875 
.868 
.866 
.863 
.862 
.861 

29.941 

•  938 
•  936 

.938 
•  94  1 

29.616 

.6i5 

.6i5 
.6i5 
.616 
.621 

29.804 
.800 
.798 

•795 

•794 

•794 

29.785 
.778 
.773 
.770 
.768 
.768 

C-^tO  ~vT  CQ  (N  1-1 
tO  to  to  tO  to  to 

Os  Os  OS  OS  Os  0s 
Os 

tN 

763 
765 
.766 
.766 
.766 

6 

7 
8 
9 
10 
Ji 

,6o5 
.626 
.644 
.652 
.652 
.638 

.771 
.787 
.8o3 
.8i5 
.816 
.8o4 

.870 
.888 

•9*7 
.926 
.980 
.926 

.961 

.967 

•97° 
.968 
.960 

.63i 
.639 
.646 
.648 
.648 
,64i 

.796 
.800 
.8o4 
.807 
.810 
.807 

.771 

•111 
.783 
.789 
.792 
.791 

.953 
.954 

•957 
.960 
.961 
.961 

.766 
.766 
.762 
.762 
.764 
.764 

Noon 
1  P.M 
2 
3 
4 
5 

.621 
.602 

.589 
.573 
.568 

.579 

.787 
.764 
•  744 
.73i 

!?3i 

.906 
.891 
.859 
.848 
.839 
.84o 

.943 
.927 
.915 
.908 
.908 
.910 

.629 
.618 
.608 
.6o5 
.6o3 
.6o4 

.806 
.798 
.791 
.786 
.783 

.782 

.786 
.781 
.776 

•774 
.772 
.771 

•959 
•  956 
.955 
.954 
.953 
.953 

,763 
•7^9 
•759 
.76i 
.763 
.765 

6 
7 
8 
9 
10 
11 

.595 
.6o4 
.617 
.636 
.64o 
,64o 

!756 

.772 
.788 

•799 

.810 

.843 
.844 
.865 
.892 
.895 
.886 

.916 
.925 
.934 

^945 

.608 
.611 
.616 
.620 
.620 
.620 

•789 

•796 
.801 

.8o5 

.8o5 

•774 
.780 
,785 
,788 
•79° 
•789 

.954 
.955 

•957 
•957 
.958 
.958 

•767 
.768 

•769 
.770 

•77' 
.769 

Mean 

29.616 

29.777 

29.877 

29.939 

29.621 

29-797 

29.780 

29.956 

29.  765 

TABLE   XII. 

DEPRESSION  OF  MERCURY  IN  GLASS  TUBES. 


Diameter 
of  Tube. 

Ivory. 

Young. 

Laplace. 

Poisson. 

Caven- 
dish. 

Pouillet. 

DanielL, 
Boiled 
Tubes. 

Diameter 
of  Tube. 

Inch. 

Inch. 

Inch. 

Inch. 

Inch. 

Inch. 

Inch. 

Inch. 

Inch. 

o.o5 

0.2949 

0.2964 

O. 

0.2796 

O. 

o. 

O. 

o.o5 

.  10 

.i4o4 

.i4a4 

,i394 

.1867 

.  i4o 

.  I  Sgo 

.070 

.  IO 

.15 

.0865 

.0880 

,o854 

,o83o 

.092 

,o858 

.044 

.i5 

.20 

,o583 

.0589 

.o58o 

.o559 

.067 

.o58o 

.029 

.20 

.25 

.0409 

.0404 

,o4i2 

.o394 

.o5o 

.0407 

.020 

.25 

.3o 

.0293 

.0280 

.0296 

.0281 

.o36 

.0296 

.014 

.3o 

.35 

.0212 

.0196 

.0216 

,O2O4 

.025 

.0216 

.010 

.35 

.4o 

.oi54 

.0139 

.oiSg 

.0149 

.oi5 

.oiSg 

.007 

.4o 

.45 

.0112 

.otoo 

.0117 

.OIO9 

.010 

.0117 

.oo5 

.45 

.5o 

.0082 

.0074 

.0087 

.0080 

.007 

.0086 

.oo3 

,5o 

.60 

,oo43 

.0045 

,oo46 

.oo4i 

.006 

.0047 

.002 

.60 

.70 

.002  3 

.0024 

.0020 

.002  5 

.70 

.80 

.0012 

.0018 

.0010 

.00  1  3 

.80 

264       TABLE   XIII. — DRY   AND   SATURATED  AIR  COMPARED. 


TABLE   XIII. 

TO    COMPARE   THE  WEIGHT    OP   A    CUBIC    FOOT    OF   DRY  AIR  AND    OP    BAT« 

URATED    AIR. 


Temp. 

Dry  Air. 

Saturated. 

Kxcess. 

Temp. 

Dry  Air. 

Saturated.       Kxcess 

Degrees. 

Grains. 

Grains. 

Grains. 

Degrees. 

Grains. 

Grains. 

Grains. 

O 

6o3.2I 

602.77 

o.44 

45 

548.16 

546.06 

2  .  IO 

I 

601.87 

601  ,4o 

o.4? 

46 

547.05 

544.88 

2.17 

2 

6OO.52 

600.  o3 

0.49 

47 

545.97 

543.75 

2.22 

3 

699.20 

598.69 

o.5i 

48 

544.85 

542.55 

2.30 

4 

597.87 

597.34 

o.53 

49 

543.75 

54i.36 

2.39 

5 

596.55 

596.01 

o.54 

5o 

542.65 

54o.2i 

2.44 

6 

5g5.24 

594.69 

o.55 

5i 

54i.55 

539.04 

2.5l 

7 

593.94 

593.36 

o.58 

52 

54o.48 

537.87 

2.61 

8 

592.63 

592.04 

0.59 

53 

53g.4i 

536.71 

2.70 

9 

Sgi.SS 

590.72 

0.61 

54 

538.33 

535.55 

2,78 

10 

5go.  04 

589.4o 

o.64 

55 

537.27 

534.39 

2.88 

1  1 

588.75 

588.07 

0.68 

56 

536.  19 

533.22 

2.97 

12 

587.48 

586.78 

0.70 

57 

535.12 

532.o6 

3.o6 

i3 

586.21 

585.49 

o.  72 

58 

534.07 

530.92 

3.i5 

i4 

584.93 

584.i8 

0.76 

59 

533.o3 

529.77 

3.26 

i5 

583.  67 

582.89 

0.78 

60 

53i  .97 

528.62 

3.35 

16 

582.  4i 

58t.6i 

0.80 

61 

530.93 

527.  48 

3.45 

17 

58i.i5 

58o.33 

0.82 

62 

529.88 

526.32 

3.56 

18 

579.91 

579.06 

o.85 

63 

528.84 

525.  17 

3.67 

:9 

578.67 

577.79 

0.88 

64 

527.81 

524.o3 

3.78 

20 

577-44 

576.54 

0.90 

65 

526.78 

522  .90 

3.88 

21 

576.21 

575.27 

0.94 

66 

525.76 

52i  .75 

4.oi 

22 

574.98 

574.01 

°;97 

67 

524.76 

52o.6i 

4.i4 

23 

573.76 

672.76 

i  .00 

68 

523.72 

519.46 

4.26 

24 

572.55 

571  .5o 

i  .o5 

69 

522.70 

518.29 

4.4i 

25 

571.33 

570.26 

i  .07 

7° 

521  .70 

517-17 

4.53 

26 

570.13 

569.01 

I  .  12 

71 

520.70 

5  i  6.  02 

4.68 

27 

568.  92 

567.77 

i.i5 

72 

519.69 

5i4.87 

4.82 

28 

567.73 

566.53 

I  .20 

73 

518.70 

5i3.75 

4.95 

29 

566.54 

565.3i 

I  .23 

74 

617.70 

5i2.6i 

5.09 

3o 

565.35 

564.o8 

I  .27 

75 

516.71 

5n.46 

5.25 

3i 

564-17 

562.86 

i.3i 

76 

5i5.73 

5io.  32 

5.4i 

32 

563.  oo 

56i.64 

1.36 

77 

5i4.?4 

509.18 

5.56 

33 

56i.84 

56o.42 

i  .42 

78 

5i3.77 

5o8.o4 

5.73 

34 

560.67 

559.20 

i.47 

79 

5i2.8o 

506.91 

5.89 

35 

55g.  5i 

558.01 

i  .  5o 

80 

5n.82 

5o5.74 

6.08 

36 

558.35 

556.79 

i.56 

81 

510.87 

5o4-6i 

6.26 

37 

557.21 

555.6i 

i  .60 

82 

509.89 

5o3.45 

6.44 

38 

556.o5 

554.4o 

i.65 

83 

508.93 

5o2.  32 

6.61 

39 

554.91 

553.20 

1.71 

84 

507.97 

5oi  .  16 

6.81 

4o 

553.77 

552.  oo 

1.77 

85 

607.  o3 

5oo.o5 

6.98 

4i 

552.65 

55o.8o 

1.84 

86 

506.07 

498.87 

7  .20 

42 

55i.52 

549.  63 

i  .  89 

87 

5o5.  ii 

497.71 

7-4o 

43 

55o.3g 

548.44 

i  .95 

88 

5o4.  19 

496.58 

7.61 

44 

549.27 

547.26 

2.OI 

89 

5o3.25 

495.44 

7.81 

[    45 

548.i6 

546.o6 

2.  10 

90 

502.32 

494.28 

8.o4 

TABLE  XIV. — HEIGHT  OF  BAKOMETER,  ETC. 


265 


TABLE   XIV. 

HEIGHT  OF  BAROMETER  CORRESPONDING  TO  TEMPERATURE  OP  BOILING  WATER 


Temp. 

Harom. 

Temp.      Barom. 

Temp. 

Barom. 

Temp. 

Barom. 

Temp.      Barom. 

O 

Inches. 

0 

Inches. 

o 

Inches. 

O 

Inches. 

O 

Inches. 

188.0 

18.  197 

igS.O 

2O.  253 

198  .0 

22.5oi 

2O3.O 

24.952 

208.0 

27.622 

.  I 

.236 

.  I 

.296 

.  I 

.548 

.  I 

25.oo3 

.  I 

.678 

.2 

.276 

.2 

.339 

.2 

.595 

.2 

.o55 

.2 

.733 

.3 

.3i5 

.3 

.382 

.3 

.642 

.3 

.  1  06 

.3 

.789 

.4 

.355 

.4 

.426 

.4 

.689 

.4 

.i58 

.4 

.845 

.5 

.395 

.5 

•  469 

.5 

•  736 

.5 

.210 

.5 

.901 

.6 

.434 

.6 

.5l2 

.6 

.784 

.6 

.26l 

.6 

•957 

.  7 

•  4?4 

•7 

.556 

•7 

.83i 

•7 

.3i3 

•7 

28.oi3 

.8 

.5x4 

.8 

•599 

.8 

.879 

.8 

.365 

.8 

.069 

•9 

.554 

•9 

.643 

•9 

.926 

•9 

.417 

•9 

.126 

189.0 

•  594 

194.0 

.687 

199.0 

•974 

2O4.0 

•  469 

209.0 

.182 

i 

.634 

.1 

.73i 

.  i 

23.  O22 

.  I 

.521 

.  i 

.239 

2 

•  674 

.2 

.775 

.2 

.070 

.2 

.573 

.2 

.295 

.3 

.714 

.3 

.819 

.3 

.118 

.3 

.626 

.3 

.352 

.4 

.755 

.4 

.863 

.4 

.166 

.4 

.678 

.4 

.409 

.5 

•795 

.5 

.907 

.5 

.214 

.5 

.780 

.5 

.466 

.6 

.835 

.6 

.gSi 

.6 

•262 

.6 

.783 

.6 

.523 

•7 

.876 

•7 

.996 

•7 

.3n 

•7 

.836 

•7 

.58o 

.8 

.917 

.8 

21  .040 

.8 

.359 

.8 

.888 

.8 

.637 

•9 

•957 

•9 

.o84 

•9 

.407 

•9 

•  94i 

•9 

.695 

190.0 

.998 

igS.o 

.  129 

200.  o 

.456 

2O5.0 

•994 

2IO.O 

.752 

.  i 

19.039 

.  i 

.174 

.  i 

.5o5 

.  I 

26.047 

.1 

.810 

.2 

.080 

.2 

.218 

.2 

.553 

.2 

.  IOO 

.2 

.867 

.3 

.  121 

.3 

.263 

.3 

.602 

.3 

.i53 

.3 

.925 

.4 

.l62 

.4 

.3o8 

.4 

.65i 

.4 

.206 

.4 

.983 

.5 

.203 

.5 

.353 

.5 

.700 

.5 

.259 

.5 

29.041 

.6 

.244 

.6 

.398 

.6 

•749 

.6 

.3:3 

.6 

.099 

•7 

.285 

•7 

.443 

•7 

.798 

•7 

.366 

•7 

.I57 

.8 

.326 

.8 

.488 

.8 

.84? 

.8 

.420 

.8 

.2l5 

•9 

.368 

•9 

.533 

•9 

.897 

•9 

.473 

•9 

.274 

191  .0 

.409 

196.0 

.578 

201  .O 

•  946 

206.0 

.527 

21  I  .O 

.332 

.  i 

.45o 

.  i 

.623 

.  I 

.996 

.  i 

.58i 

.1 

.391 

.2 

.492 

.2 

.669 

.2 

24.o45 

.2 

.635 

.2 

.449 

.3 

.534 

.3 

.?i4 

.3 

.095 

.3 

.689 

.3 

.5o8 

.4 

.575 

.4 

.760 

.4 

.i45 

.4 

•  743 

.4 

.567 

1        -5 

.617 

.5 

.806 

.5 

.  ig5 

.5 

•797 

.5 

.626 

.6 

.659 

.6 

.85i 

.6 

.245 

.6 

.852 

.6 

.685 

•7 

.701 

•7 

.897 

•7 

.295 

•7 

.906 

•7 

•  744 

.8 

•  743 

.8 

.943 

.8 

.345 

.8 

.961 

.8 

.8o3 

•9 

.785 

•9 

.989 

•9 

.395 

•9 

27.015 

•9 

.863 

192.0 

.827 

197.0 

22.035 

202  .0 

.445 

207.0 

.070 

212.0 

.922 

.  i 

.869 

.  i 

.081 

.  i 

•  4g5 

.  i 

.125 

.  I 

.982 

.2 

.912 

.2 

.128 

.2 

.546 

.2 

.180 

.2 

3o.o4i 

.3 

.954 

.3 

.  174 

.3 

.596 

.3 

.235 

.3 

.  101 

.4 

.996 

•  4 

.221 

.4 

.64? 

.4 

.290 

.4 

.161 

.5 

20.039 

.5 

.267 

.5 

.697 

.5 

.345 

.5 

.221 

.6 

.082 

.6 

.3x4 

.6 

.748 

.6 

•  4oo 

.6 

.281 

•7 

.  \"2.t\ 

•7 

.36i 

-7 

•799 

•7 

.456 

•7 

.341 

.8 

.167          .8 

.407 

.8 

.85o 

.8 

.5n 

.8 

.4oi 

•9 

.210          .9 

.454 

.0 

.QOI 

•Q 

.566 

.0 

.462 

266      TABLE  XV. — DIURNAL  VARIATION  OF  TEMPERATURE. 


TABLE    XV. 

DITJKNAL  VARIATION  OF  TEMPERATURE  AT   NEW  HAVEN,  CONNECTICUT. 


Hours. 

Jan. 

Feb. 

Mar. 

Apr. 

May. 

June. 

July. 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Year. 

o 

o 

o 

o 

0 

0 

o 

o 

o 

o 

o 

o 

0 

Midn'ht 

2.3 

2.9 

3.7 

4.6 

5.4 

5.8 

5.2 

4-7 

4.8 

4.i 

2.6 

2.2 

4.0 

i 

2.6 

3.3 

4.3 

5.4 

6.3 

6.9 

6.2 

5.6 

5.6 

4.8 

3.2 

2.5 

4-7 

2 

3.o 

3.8 

4.8 

6.1 

7.2 

8.0 

7.0 

6.3 

6.3 

5.5 

3.7 

2.8 

5.4 

3 

3.3 

4.3 

5.4 

6.7 

8.0 

8.7 

7.5 

6.8 

6.8 

6.0 

4.i 

3.2 

5.9 

4 

3.7 

4.8 

5.8 

7.3 

8.5 

8.9 

7-7 

7.2 

7.2 

6.5 

4.5 

3.5 

6.3 

5 

4-1 

5.2 

6.2 

7.5 

8.4 

8.2 

7-4 

7-i 

7.3 

6.8 

4.8 

3.8 

6.4 

6 

4.3 

5.3 

6.1 

7-1 

6.6 

6.1 

6.1 

6.4 

6.8 

6.6 

4.8 

4.o 

5.9 

7 

4.4 

5.i 

4.9 

5.3 

3.6 

3.2 

3.7 

4.i 

4.7 

5.3 

4.5 

4.o 

4.4 

8 

3.8 

3.7 

2.3 

2.0 

o.5 

0.0 

0.9 

i.3 

i-7 

2.3 

3.o 

3.2 

2.1 

9 

1.3 

o.5 

-o.5 

-I  .  I 

-2.  I 

-2.7 

-1.6 

-I  .2 

-I  .2 

-0.6 

o.5 

I  .0 

-0.6 

10 

-1.6 

-2.5 

-3.2 

-3.9 

-4.2 

-4.7 

-3.8 

-3.4 

-3.8 

-3.5 

-2.2 

-1.6 

-3.2 

ii 

-3.6 

-4.2 

-4.9 

-5.5 

-5.8 

-6.1 

-5.6 

-5.3 

-5.6 

-5.6 

-4.2 

-3.5 

-5.o 

Noon 

-5.2 

-5.6 

-6.1 

-6.8 

-7.0 

-7-i 

-6.7 

-6.5 

-6.9 

-7.0 

-5.6 

-5.0 

-6.3 

* 

-6.1 

-6.6 

-7.0 

-7-7 

-7-9 

-7-9 

-7.5 

-7.3 

-7-7 

-7-7 

-6.4 

-5.8 

-7.1 

2 

-6.3 

-6.9 

-7.5 

-8.3 

-8.5 

-8.3 

-7.8 

-7-7 

-8.0 

-8.1 

-6.6 

-6.1 

-7.5 

3 

-5.9 

-6.8 

-7.3 

-8.3 

-8.5 

-8.2 

-7-7 

-7.6 

-7-9 

-7-7 

-6.2 

-5.5 

-7.3 

4 

-4-7 

-5.8 

-6.6 

-7.8 

-8.0 

-7.6 

-7.2 

-7-i 

-7.1 

-6.6 

-4.6 

-4.o 

-6.4 

5 

-2.8 

-3.8 

-4-7 

-6.6 

-6.8 

-6.5 

-6.1 

-5.9 

-5.8 

-4.5 

-2.9 

-2.1 

-4.9 

6 

-i.4 

-2.O 

-2.5 

-4.0 

-4-7 

-4.3 

-4.2 

-3.9 

-4.0 

-2.8 

-1.6 

-I  .0 

-3.0 

7 

-o.3 

-0.6 

-0.9 

-i.5 

-1.6 

-2.2 

-2.O 

-1.9 

-1.9 

-I  .2 

-o.5 

-O.2 

-I  .2 

8 

o.5 

0.4 

0.6 

0.6 

0.6 

O.  I 

-O.  I 

O.O 

O.  I 

0.2 

o.4 

o.5 

0.3 

9 

i  .  i 

I  .2 

!-7 

2.0 

2.2 

1.8 

1.6 

1.6 

'•7 

1.5 

i  .  i 

I  .0 

i.5 

10 

i.5 

1.8 

2.4 

3.o 

3.5 

3.3 

2.9 

2.8 

2.8 

2.4 

1.6 

i.5 

2.5 

ii 

1.9 

2.4 

3.o 

3.8 

4.4 

4.6 

4.i 

3.9 

3.9 

3.3 

2.  I 

1.8 

3.3 

Da.  ext. 

-I.O 

-0.8 

-0.6 

-o.4 

O.O 

o.3 

-O.  I 

-o.3 

-o.3 

-0.6 

-0.9 

-i  .  i 

-0.6 

7-i 

-0.8 

-0.7 

-i  .1 

-I  .2 

-2.  I 

-2.4 

-1.9 

-1.6 

-i.5 

-1.2 

-0.9 

-0.9 

-i.4 

7.2 

-I  .0 

-0.9 

-1.3 

-1.5 

-2.4 

-2.6-2.1 

-1.8 

-1.6 

-i.4 

-I  .0 

-i  .  i 

-1.6 

8.1 

-i  .  i 

-i.4 

-2.4 

-2.8 

-3.7 

-4.0-3.3 

-3.o 

-3.o 

-2.7 

-I'7 

-i.3 

-2.5 

8.2 

-i.3 

-1.6 

-2.6 

-3.i 

-4.0 

-4.2 

-3.5 

-3.2 

-3.2 

-2.9 

-1.8 

-i.4 

-2.7 

9.8 

0.9 

o.5 

0.0 

-O.2 

-0.8 

-i.3 

-0.9 

-0.6 

-0.6 

-O.2 

o.4 

0.7 

-0.2 

6.6 

1.5 

1.6 

1.8 

i.5 

0.9 

0.9 

i  .0 

I  .2 

i.4 

I9 

1.6 

i.5 

i.4 

7-7 

2.0 

2.2 

2.O 

1.9 

I  .0 

o.5 

0.8 

I  .  I 

i.4 

2.O 

2.0 

1.9 

1.6 

8.8 

2.  I 

2.  I 

i.4 

1.3 

0.6 

O.O 

o.4 

0.7 

0.9 

I  .2 

I'7 

1.8 

I  .2 

9.9 

I  .2 

0.9 

0.6 

o.4 

O.O 

-0.4 

O.O 

0.2 

O.2 

0.4 

0.8 

I  .0 

0.4 

10.  10 

O.O 

-0.3 

-0.4 

-0.4 

-0.4 

-0.7 

-o.4 

-0.3 

-0.5 

-0.5 

-o.3 

-O.I 

-0.4 

6.2.6 

-I  .  I 

-I  .2 

-i.3 

-i-7 

-2.2 

-2.2 

-1.9 

-1.8 

-i-7 

-i.4 

-i.i 

-1  .0 

-1.6 

6.2.8 

-0.5 

M).4 

-o.3 

-0.2 

-0.4 

-0.7 

-0.6 

-o.4 

-o.4 

-0.4 

-o.5 

-o.5 

-0.4 

6.2.9 

-o.3 

-O.  I 

0.  I 

o.3 

0.  I 

-O.  I 

O.O 

O.I 

O.2 

O.O 

-O.2 

-0.4 

O.O 

6.2.  10 

-O.  I 

O.  I 

o.3 

0.6 

o.5 

0.4 

0.4 

o.5 

o.5 

o.3 

-O.I 

-O.2 

o.3 

7.2.9 

-o.3 

—  O.2 

-o.3 

-o.4 

-0.9 

-i  .1 

-0.8 

-0.6 

-o.5 

-o.4 

-0.3 

-0.4 

-o.5 

7.2.9.0    o.i 

0.  I 

O.2 

0.2 

—  O.  I 

—  O  .  4  '—  O  .  2 

—  O.  I 

O.O 

O.O 

O.O 

O.O 

0.0 

TABLE   XVI. — DIUKNAL   VARIATION   OF  TEMPEKATUKE.     267 


TABLE   XVL 

DIURNAL  VARIATION  OF  TEMPERATURE   AT   GREENWICH,  ENGLAND. 


Hours. 

Jan. 

Feb. 

Mar. 

Apr. 

May. 

June. 

July. 

Aug. 

Sept 

Oct 

Nov. 

Dec. 

Year. 

o 

O 

o 

o 

O 

o 

O 

o 

o 

o 

o 

o 

O 

Midn'ht 

1  .0 

1.6 

2.9 

4.8 

5.4 

6.2 

5.o 

5.i 

4.0 

2.9 

1-7 

0.9 

3.5 

i 

0.9 

1.8 

3.0 

5.2 

6.0 

7-1 

5.5 

5.5 

4.5 

3.o 

1.8 

I.O 

3.8 

2 

I  .2 

2.O 

3.3 

5.7 

6.4 

8.0 

6.0 

6.0 

5.5 

3.4 

2.O 

I  .2 

4.2 

3 

i.3 

2.  I 

3.6 

6.2 

6.7 

8.7 

6.4 

6.3 

6.4 

3.6 

2.O 

1.3 

4.5 

4 

1.6 

2.3 

3.9 

6.6 

6.7 

9.3 

6.6 

6.5 

6.6 

3.8 

2.  I 

i.4 

4.8 

5 

1.8 

2.2 

4.0 

6.7 

6.3 

8.8 

6.2 

6.5 

6.2 

3.8 

2.0 

1-4 

4-7 

6 

1.9 

2.3 

3.9 

6.0 

4.8 

6.4 

4.5 

5.5 

5.3 

3.5 

I.9 

i.4 

3.9 

7 

i  .9 

2.  I 

3.6 

4.3 

2.6 

3.o 

2.5 

3.3 

4.0 

2.8 

'•7 

i.5 

2.8 

8 

i  .5 

1.6 

2.5 

2.0 

o.5 

o.o 

o.o 

0.9 

2.  I 

1.6 

I  .0 

i.3 

I  .2 

9 

I  .0 

0.7 

0.2 

-0.9 

-2.0 

-2.5 

-2.0 

-1.6 

-0.4 

o.o 

o.4 

0.9 

-0.5 

10 

O.2 

-o.5 

-1.9 

-3.2 

-4.0 

-4.5 

-4.0 

-3.5 

-3.o 

-2.O 

-0.6 

o.o 

-2.2 

1  1 

-i.3 

-2.  I 

-3.5 

-5.3 

-5.5 

-5.8 

-5.4 

-5.4 

-5.o 

-3.8 

-2.0 

-i.3 

-3.9 

Noon 

-2.3 

-3.2 

-5.o 

-6.8 

-6.7 

-7.3 

-6.4 

-6.5 

-6.4 

-5.i 

-3.i 

-2.  I 

-5.i 

i 

-2.9 

-3.9 

-5.8 

-7-9 

-7.5 

-8.1 

-6.7 

-7.5 

-7.1 

-5.5 

-3.5 

-2.4 

-5.7 

2 

-3.0 

-3.9 

-5.8 

-8.2 

-7.7 

-8.6 

-6.7 

-7-7 

-7.1 

-4.9 

-36 

-2.3 

-5.8 

3 

-2.5 

-3.6 

-5.5 

-7-7 

-7.3 

-8.4 

-6.5 

-7.0 

-6.6 

-3.7 

-3.o 

-1-9 

-5.3 

4 

-1.9 

-2.8 

-4.5 

-6.7 

-6.1 

-7-4 

-5.8 

-5.5 

-5.5 

-2.8 

-2.1 

-1.3 

-4-4 

5 

-1  .  1 

-1.6 

-3.3 

-5.4 

-4.8 

-6.1 

-4.9 

-3.6 

-4.2 

-1.7 

-1.2 

-0.8 

-3.2 

6 

-0.6 

-0.6 

-1.8 

-3.5 

-3.o 

-4.5 

-3.5 

-2.O 

-2.5 

-0.8 

-0.4 

-o.4 

-2.0 

7 

-o.3 

o.3 

-o.4 

-i  .  i 

—  I.O 

-2.4 

-i.5 

-0.5 

-0.6 

o.o 

O.I 

-0.  I 

-0.6 

8 

O.  I 

0.6 

0.9 

0.7 

0.9 

o.o 

o.3 

1  .0 

i  .0 

0.7 

0.6 

0.2 

0.6 

9 

o.4 

i  .0 

i-7 

2.0 

2.3 

1.8 

1,9 

2.4 

1.8 

i.3 

I  .0 

0.4 

i.5 

10 

0.6 

1.3 

2.3 

3.2 

3.5 

3.6 

3.3 

3.3 

2.7 

1.9 

1.3 

o.5 

2.3 

1  1 

0.7 

1.5 

2.6 

4-1 

4.5 

5.o 

4.2 

4.3 

3.4 

2.4 

i.5 

0.8 

2.9 

Da.  ext. 

-o.5 

-0.8 

-0.9 

-0.7 

-o.5 

o.3 

-0.  I 

-0.6 

-o.3 

-0.8 

-0.7 

-o.4 

-o.5 

7-i 

-o.5 

-0.9 

-i  .  i 

-1.8 

-2.4 

-2.6 

-2.  I 

-2.  I 

-i.5 

-i.4 

-0.9 

-o.4 

-i.5 

7.2 

-o.5 

-0.9 

-i  .  i 

-1.9 

-2.5 

-2.8-2.1 

-2.2 

-i.5 

-1  .0 

-0.9 

-o.4 

-i.5 

8.1 

-0.7 

-i  .  i 

-1.6 

-2.9 

-3.5 

-4.0 

-3.4 

-3.3 

-2.5 

-1.9 

-i.3 

-o.5 

-2.2 

8.2 

-0.7 

-i  .  i 

-1.6 

-3.i 

-3.6 

-4.3 

-3.3 

-3.4 

-2.5 

-1.7 

-i.3 

-o.5 

-2.3 

9.8 

o.5 

0.6 

o.5 

-O.I 

-o.5 

-I  .2 

-0.8 

-o.3 

o.3 

o.3 

o.5 

o.5 

O.O 

6.6 

0.6 

0.9 

i  .0 

I  .2 

0.9 

0-9 

o.5 

'•7 

r.4 

1.3 

0.8 

o.5 

0.9 

7-7 

0.8 

I  .2 

1.6 

1.6 

0.8 

o.3 

o.5 

i.4 

i-7 

i.4 

0.9 

0.7 

I  .  I 

8.8 

0.8 

I  .  I 

J-7 

i.3 

0.7 

0.0 

0.  I 

0.9 

i.5 

i  .  i 

0.8 

0.8 

0.9 

9.9 

0.7 

0.8 

0.9 

o.5 

O.I 

-0.3 

o.o 

0.4 

0.7 

0.6 

0.7 

0.6 

o.5 

10.  10 

o.4 

0.4 

0.2 

o.o 

—  O.2 

-0.4 

-0.4 

-0.  I 

-O.  I 

o.o 

o.4 

O.2 

o.o 

6.2.6 

—  0.6 

-0.7 

-I  .2 

-1.9 

-I.9 

-2.2 

-1.9 

-i.4 

-i.4 

-0.7 

-0.7 

-0.4 

-1.3 

6.2.8 

-o.3 

-0.3 

-0.3 

-o.5 

-0.7 

-0.7 

-0.6 

-0.4 

-o.3 

-O.2 

-o.4 

-O.2 

-o.4 

6.2.9 

—  O.  2 

-O.I 

-0.  I 

-O.I 

-O.2 

-O.  I 

-0.  3 

o.o 

0.0 

O.O 

-O.2 

-O.2 

-O.I 

6.2.  IO 

-0.2 

-O.  I 

O.  1 

o.3 

O.2 

o.5 

0.4 

o.3 

o.3 

O.2 

-0.  I 

-O.I 

O.I 

7.2.9 

-0.2 

-o.3 

—  O.2 

—  0.6 

-0.9 

-I  .2 

-0.8 

-0.7 

-o.4 

-0.2 

-0.3 

-O.I 

-o.5 

7.2.9.9 

—  O.  I 

O.  I 

o.3 

0.0 

—  O.  I 

—  o.  5 

-0.  I 

—  O.  I 

0.  I 

O.I       0.0 

O.O 

o.o 

268 


TABLE   XVII. — MEAN   TEMPERATURES 


TABLE   XVII. 

MEAN   TEMPEKATURES  FOR  EACH  MONTH,  SEASON,  AND  THE  TEAR. 


Place. 

Lat. 

Long. 

Alt. 

Jan. 

Feb. 

March. 

April. 

Paramaribo,  Dutch  Guiana  .  . 
St  Vincent  West  Indies  

O       ' 

5  44 
i3  10 

O       ' 

55   i3 
60  3i 

Feet. 

o 
78.2 
80.0 

0 

78.0 

70.  3 

O 
78.9 
7O  .  7 

0 

79.2 

81  .3 

17  58 

76  5o 

5o 

75.7 

76.0 

?5  .  Q 

78.1 

Vera  Cruz  Mexico    

IQ     12 

06       Q 

5o 

70.  o 

71.6 

73.4 

77  .  2 

Mexico  City  

19  25 

99     6 

6990 

52.5 

56.4 

61.  i 

63.o 

Havana  Cuba             

23     9 

82  23 

5o 

71  .4 

?4.o 

74.  1 

76.6 

Key  West,  Florida  

24    32 

81  47 

10 

68.1 

60.4 

72.7 

75.3 

Galveston  Texas    

29   18 

Q.4  4? 

54.2 

60.  2 

60.2 

71.6 

St.  Augustine,  Florida  

29  48 

81   35 

20 

57.0 

5o  .  o 

63.3 

68.8 

New  Orleans,  Louisiana  

29  57 

90     o 

10 

54.8 

56.4 

62.9 

69.0 

Bermuda,  Atlantic  Ocean  .... 
San  Diego,  California  

32    20 
32    42 

64  5o 
117   i3 

i5o 

56.8 
5i  .9 

58.6 
53.3 

59.4 
56.o 

b'2.8 

6l  .2 

Charleston,  South  Carolina.  .  . 
Santa  Fe  New  Mexico  

32  46 
35  4i 

79  56 
1  06     i 

20 

68/16 

5o.3 
3i.4 

52.4 
33.2 

58.7 

4o.  7 

65.4 
5i.3 

Richmond,  Virginia  

37  32 

77  27 

120 

33.7 

39.8 

47.  1 

54-7 

San  Francisco,  California  .... 
St.  Louis,  Missouri  

37  48 
38  37 

122    26 

90  1  5 

1OO 

rf5o 

49.8 
32.9 

5a.o 
35.o 

53.7 
44.4 

57.0 
58.3 

Washington,  D.  C  

38  53 

77     ° 

80 

34.i 

36.7 

45.3 

55.7 

Cincinnati  Ohio  

3g     6 

84  3o 

5/J3 

33.i 

34.i 

43.5 

54.i 

Baltimore,  Maryland  

3g   1  8 

76  37 

36 

32.8 

34.2 

42.3 

52.7 

Philadelphia,  Pennsylvania  .  . 
New  York  City,  New  York  .  . 
Salt  Lake  City,  Utah  

39  58 
4o  43 
4o  46 

75   10 
74     o 

112       6 

4o 

23 

dT>i 

3i.8 

30.2 

27.  i 

32.3 
3o.4 
34.o 

4i  .0 
38.3 
3q.7 

5i.8 
48.6 
5o  .  2 

New  Haven,  Connecticut  .... 
Cleveland,  Ohio  

4i   18 

4i  3i 

72  55 
81   5i 

5o 
660 

26.5 
26.2 

28.1 
29.0 

36.i 
35.6 

46.8 
47-7 

Chicago,  Illinois  

4i  54 

87  38 

J>Q  1 

23.6 

24.  7 

32.3 

46.  i 

Fort  Laramie,  Dacotah  

42     12 

io4  48 

45l9 

3i  .0 

32.6 

36.8 

An     g 

Detroit,  Michigan  

42    2O 

83     2 

58o 

27  .  O 

26.6 

35  4 

46  3 

Boston,  Massachusetts  

42    21 

71     3 

5o 

27.8 

27  .  O 

36.2 

46  4 

Buffalo,  New  York  

42    5T 

78  55 

600 

23.4 

2  I  .  I 

35  5 

Toronto,  Canada  

43   4r> 

7Q    23 

34  1 

24.3 

23.  I 

3o  4 

4i    3 

Halifax,  Nova  Scotia  

44  39 

63  37 

20 

22  .6 

23.7 

So.g 

38.9 

Fort  Snelling,  Minnesota  .... 
Astoria,  Oregon  

44  53 
46  ii 

93     8 
t23  48 

820 

In 

i3.7 
43.0 

17.6 

43.6 

3i.4 
45.7 

46.3 
52.8 

Fort  Brady,  Michigan  

46  39 

84  43 

600 

17.2 

16.  2 

25.  1 

38.3 

Quebec,  Canada  

46  4o 

71     12 

3oo 

10.4 

i3.8 

26  4 

St.  Johns,  Newfoundland  .... 
Cumberland  House  

4?  33 
53  57 

52    28 
IO2    17 

1  4" 
900 

23.3 

—    7  .O 

20.9 

-  4.6 

24.2 

I  5  .  2 

33.4 

Sitka,  Aliashka  

ST     3 

i35  18 

•>o 

3o.o 

34  i 

3o  o 

Nain,  Labrador  

57   10 

61   5o 

5o 

-  2.9 

-  °-7 

7.6 

Jv  •  v 

22  .  7 

Godhaab,  Greenland  

64  i° 

52    24 

1  5   0 

Fort  Franklin,  Brit.  America  . 
Boothia  Felix,  Arctic  Regions 
Melville  Island,  Arc.  Regions  . 
Van  Rensselaer  Harbor,  A.  R. 

65   12 
69  5g 

74  4? 
78  37 

123   i3 
92     i 
no  48 

70  53 

5oo 

5 

-22.3 
-28.7 

-3i.3 

-28.2 

-16.7 

-32.0 
-32.4 

-26.4 

-  5.4 
-28.7 
-18.2 

-34.9 

12.4 

-   2.6 

-    8.2 

-io.3 

FOR   EACH   MONTH,   SEASON,  AND   THE   YEAR. 


269 


TABLE   XVII. 

MEAN   TEMPERATURES  FOB  EACH  MONTH,  SEASON,  AND   THE   YEAR. 


May. 

June. 

July. 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Spring. 

Summer. 

Autumn. 

Winter. 

Year. 

o 

79-9 
82.2 
8o.3 
80.4 
66.1 

o 
79-5 

82.2 
80.6 
81.9 

65.4 

o 
80.0 
82.2 
8t.7 
8i.5 
65.4 

o 
82.0 
82.8 
81.0 
82.4 
64-9 

o 

83.4 
83.3 
80.7 
81.0 
64.3 

o 
83.3 
82.7 
79.8 
78.4 
60.2 

o 
8l.5 
82.1 
78.7 

?5.4 
55.8 

O 

79-7 
80.4 
76.  7 
71.1 

52.  O 

o 
79.3 

81.1 
76.2 

77.0 
63.4 

O 

80.5 
82.4 
78.1 
81.9 

65.2 

o 

82.7 

82.  7 
81.1 
78.3 
60.  i 

o 
78.6 

79-9 
79-7 
70.9 
53.6 

o 
8o.3 
8l.5, 
78.8 
77.0 
60.6 

78.0 
79.0 
81.1 
73.5 

74-8 

81.0 
81.4 
83.5 
79.3 
79-9 

8i.5 
82.7 
85.6 
80.9 
81.6 

81.6 
82.8 
85.9 
8o.5 
81.2 

8o.4 
81.6 
82.7 
78.6 

78.5 

78.8 

77-7 
68.6 
71.9 
69.8 

75.i 

74-7 
60.  i 

64.i 
60.2 

73.5 
70.7 
57.5 
57.2 
56.o 

76.2 
75.7 
?4-o 
68.5 
68.9 

8i.3 
82.3 
85.o 
8o.3 
81.0 

78.1 
78.0 
7o.5 
7i.5 
69.3 

73.0 
69.7 
57.3 
58.1 
55.7 

77.2 
76.4 
71.7 
69.6 
68.7 

69.1 
62.7 
73.4 
57.i 
65.4 

73.2 

67.4 
79.0 

68.8 
73.8 

75.7 
72.7 
81.7 
72.6 
77.6 

76.6 

73.7 
80.9 
70.0 
74.8 

76.8 
70.9 
76.9 
61.9 
67.1 

73.0 
65.5 
67.9 
5i.3 
57.5 

65.8 
56.9 
59.5 
38.6 
44.2 

60.6 

5i.7 

52.5 

30.2 

38.i 

63.7 
60.0 
65.8 

49.7 
55.7 

75.2 
71.2 
80.6 
70.4 
75.4 

71.9 
64.4 
68.1 
5o.6 
56.3 

58.8 
52.3 
5i.7 
3i.6 
37.2 

67.4 
62.0 
66.6 
5o.6 
56.2 

56.5 
66.4 
66.3 
63.6 
63.i 

57.7 
74-0 
74.4 
7i.4 
71.6 

58.8 
78.5 
78.3 
76.5 
76.7 

Sg.o 
76.5 
76.3 
74.2 

74-7 

59.9 
68.7 
67.7 
66.  o 
67.8 

59.8 
55.4 
56.7 
53.2 
55.7 

55.6 
40.9 

44.8 
42.5 
45.i 

5i.3 
33.6 
37.3 
33.8 
35.6 

55.  7 
56.4 
55.8 
53.7 
52.  7 

58.7 
76.3 
76.3 
74-o 
74.3 

58.4 
55.o 
56.4 
53.9 
56.2 

5l  .2 

33.8 
36.i 
33.7 

34.2 

56.o 
55.4 
56.i 
53.8 
54.3 

62.5 
Sg.S 
63.  o 
57.3 
56.6 

7i.5 
68.3 
7i.3 
67.0 
66.3 

76.0 
74.8 
76.9 
71.7 
71.9 

73.2 

73.2 

75.0 
70.3 
68.8 

63.8 
65.8 
67.1 
62.5 
62  .4 

54.5 
54.5 
55.6 
5i.i 
49-3 

44.o 
43.3 
41.7 
4o.3 
37.9 

34.5 
33.5 
3i.3 
3o.4 
29.6 

5i.8 
48.7 
5i  .0 
46.7 
46.6 

73.6 
72.  i 
74.4 
70.0 
69.0 

54.1 
54.5 
54.8 
5i.3 
49.9 

32.9 
3i.4 
3o.8 

28.4 
28.3 

53.i 
5i.7 
52.7 
49.0 
48.5 

56.3 
56.i 
56.o 
56.5 
55.3 

62.7 
67.3 
65.6 
66.2 
67.4 

70.8 

74-7 
69.7 
71.6 
7i.5 

68.5 
73.8 
67.5 
69.4 
70.0 

bo.  i 

64.2 

60.0 

62  .2 

59.9 

48.5 
50.9 

47-7 
5i.5 

48.7 

37.9 
35.8 
38.2 
4i  .0 
37.2 

29.3 
28.0 
26.9 
Si.'i 

22.8 

44-9 
46.8 
45.9 
46.3 
43.8 

67.3 
71.9 
67.6 
69.  i 
69.6 

48.8 
5o.3 
48.7 
5i.6 
48.6 

25.9 
3i.i 
26.8 
28.9 
22.4 

46-7 
5o.  i 
47.2 
48.  9 
46.i 

sirs 

48.o 

5g.o 
55.o 
49-3 

6i.4 
56.o 
68.4 
59.5 
58.4 

66.8 
62.0 
73.4 
61.6 

64-7 

66.3 
64.4 
70.  i 
63.o 
62.9 

58.i 
58.4 
58-9 
58.7 
54.6 

45.2 

48.o 
47.1 
55.4 
43.5 

36.6 
38.5 
3i.7 
46.4 
32.5 

26.2 
27.7 
16.9 

4o.7 

21  .5 

4i.i 

39.3 
45.6 
5i.i 

37.6 

64.8 
60.8 
70.6 
61.6 
62.0 

46.6 
48.3 
45-9 
53.7 
43.5 

24.5 
24-  7 
16.1 
42.4 
i8.3 

44.3 

43.2 

44-6 

52.2 

4o.4 

53.2 
39.3 
5i.3 
46.o 
32.8 

64-5 
48.o 
58.8 
52.5 
4i.8 

69.0 
56.2 
61.8 
55.i 

48.2 

68.1 
57.9 
59.5 
55.i 
5i.i 

56.8 
53.o 
45.8 
5o.o 
42.2 

43-9 
44.5 
35.o 

44.i 

32.2 

32.9 
•34.0 
17.2 
37.7 

22.3 

i5.o 
25.3 
5.6 
35.9 
3.4 

39.5 
32.3 
32.5 
4o.o 

21  .7 

67  .2 
54-0 
60.0 
54.2 
47.o 

44.6 
43.8 
32.7 
43.9 

32.2 

i3.o 

23.2 
-2.0 
32.2 
-    0.4 

4o.6 
38.3 
3o.8 
42.6 

25.1 

32.2 

35.2 
i5.6 
16.8 
i3.4 

3g.  i 
48.o 
34.2 
36.2 
3o.  i 

41.9 

52.  I 

4i.3 
42.4 
38.2 

4o.8 
5o.6 
38.7 
32.6 
3i.8 

35.6 
4i  .0 
25.4 

22.5 
!3.4 

29.8 
22.5 
9.I 
-2.8 

-3.6 

21  .9 
-O.I 

-  5.4 

—  21  .  I 
-21.9 

i7.5 
-10.9 
-22.4 

-21.6 

-3i.i 

23.3 
14.0 

-    5.2 
-    3.2 

-10.6 

4o.6 
5o.2 
38.o 
37.i 
33.4 

29.  I 
21  .  I 

9-7 
-o.5 

-4.0 

U.i 

-16.7 

-27.7 

-28.4 
-28.6 

26.8 
17.2 
3.7 

I  .2 
-2.5 

270 


MEAN   TEMPERATURE   OF   CERTAIN   LOCALITIES. 


TABLE  XVIII.  —  PLACES  WHOSE  MEAN  TEMPERATURE  is  ABOVE  80°  FAH. 


Place. 

Latitude. 

Longitude. 

Tempera- 
ture. 

No.  ol 
Years 

Ni^er  Africa  

O        ' 

5     o 

O        / 
—      6 

O 
85.27 

I 

Maracaibo,  South  America  .... 
Kouka  Central  Africa  

10  43 
i3   10 

71  52 

—   i4  3o 

84-75 
83.63 

I 

Coburg  Peninsula,  Australia.  .  . 
Pondichery,  India  

—  ii     5 
ii   56 

—  i3a  i5 
—  TO   5a 

82.79 
8a.58 

I 

2 

Calcutta  India  

22    35 

—  88  20 

82.41 

4 

Madras,  India  

i3     4 

—    80    IQ 

8i.g4 

20 

Samarang,  Java  

—  6  5o 

—  i  10  3o 

81.87 

i 

Nagpoor  India  

21       8 

—    70    1  1 

8i.5g 

3 

Rio  Berbice,  British  Guiana  .  .  . 
St  Vincent,  West  Indies  

6  29 
i3   10 

S?    24 

Go  3i 

8i.56 
8i.52 

i 

6 

Porto  Rico,  West  Indies  

18  20 

66   1  3 

8i.38 

5 

Guinea,  Africa  

5  3o 

o     o 

8i.38 

i 

St  Domingo,  West  Indies  

18  20 

70     o 

81.20 

i 

St.  Christopher,  West  Indies  .  .  . 
Bombay  India        

17  44 
18  56 

64  49 

*72    5A 

81.27 
81  .27 

i 

i 

St  Thomas  West  Indies    

18  21 

64  56 

8i.23 

i 

Anjarakandy,  India  

1  1  4o 

—  75  4o 

8  1  .07 

I  O 

Cobbe  Africa  

i4  1  1 

—  28     8 

80.06 

2 

Colombo   Ceylon  

6  57 

—  80     o 

80.75 

I 

Trincomalee  Ceylon  

8  34 

—    8l     22 

80.75 

2 

Demerara,  British  Guiana  

6  45 

58     2 

80.71 

I 

Para  Brazil  

—   i  28 

48  29 

80.  70 

4 

Singapore  Malacca  

I     17 

—  io3  5o 

80.68 

6 

Upper  Park  Camp,  Jamaica.  .  . 

17  58 

76  5o 

8o.63 

i 

Fort  Dundas  Australia  

—  II     25 

—  l32    25 

80.  63 

i 

Christiansborg,  Africa  

5  24 

—     o  16 

80.42 

4 

Paramaribo,  Dutch  Guiana.  .  .  . 
Benares  India  

5  44 

25    18 

55   i3 
—  82  56 

80.  3o 
80.26 

2 

3 

Kingstown,  West  Indies  

i3     8 

60  37 

8o.25 

I 

Cawnpore,  India  

26    20 

—    80    22 

80.21 

I 

Upper  Egypt  

26     o 

—  33  4o 

80.  i  o 

I 

TABLE  XIX.  —  PLACES  WHOSE  MEAN  TEMPERATURE  is  BELOW  18°  FAH. 


Place. 

Latitude. 

Longitude. 

Tempera- 
ture. 

No.  of 
Years. 

Van  Rensselaer  Harbor  

O        ' 

78  37 

O       / 

70  53 

O 
—2.46 

2 

Melville  Island  

74  4? 

1  10  48 

+  1.24 

I 

Ustjansk,  Siberia  

70  55 

—  i38  24 

2.75 

2 

Port  Bowen,  Arctic  Regions  .  .  . 
Boothia  Felix,  Arctic  Regions.  . 

?3  i4 
§9  5g 

88  56 
92     i 

3.53 
3.70 

I 

Igloolik,  North  America  

69  21 

81   53 

5.55 

I 

Fort  Hope,  North  America.  .  .  . 
Winter  Island  

62  32 
66  ii 

86  56 
83  ii 

6.  10 

8.82 

I 

Nishne  Kolymsk,  Siberia  
Jakutsk,  Siberia  

68  32 

62       2 

—  i  60  56 
—129  44 

g.So 
H.53 

2 

I? 

Fort  Enterprise,  North  America 
Karische  Pforte,  Nova  Zembla  . 
Yucon,  Russian  America  

64  20 
70  36 
66     o 

n3     6 

—  S?  47 
i4?    o 

13.90 
14.90 
16.80 

3 
i 

i 

Matoshkin  Schar,  Nova  Zembla 
Fort  Franklin,  Great  Bear  Lake 
Fort  Churchill,  Hudson  Bay  .  .  . 

73   19 
65   12 
5g     2 

—    57    20 

123   i3 
93   10 

16.93 
17.18 
17-45 

i 

2 
2 

TABLES  XX.,  XXI. — MONTHLY  RANGE   OF  TEMPERATURE.    271 


TABLE  XX. — PLACES  HAVING  A  SMALL  MONTHLY  RANGE  OF  TEMPERATURE. 


Place. 

Latitude. 

Longitude. 

Hottest 
Month. 

Coldest 
Month. 

Differ- 
ence. 

No.  of 
Years. 

Coinmewine,  South  America.  . 
Buitenzorg,  Java  

O        ' 

5  38 
—  6  37 

o        / 

54  42 
—  i  06  49 

0 

79.2 

77.8 

O 
77.0 
75.2 

o 

2.2 
2.6 

2 
3 

Souttea,  Asia  

8l.2 

78.5 

2.7 

I 

Puerto  d'Espana,  S.  America  . 
Singapore  Asia    

10  38 

I     17 

61   34 
—  io3  5o 

79.5 
82.2 

76.5 

78.5 

3.o 

3.7 

I 

6 

Kingstown  St  Vincent  

i3     8 

60  37 

81.8 

78.1 

3.7 

i 

Kandv  Ceylon          

717 

—  80  4g 

74.6 

7O.6 

4.o 

3 

St.  Vincent,  West  Indies  

i3   10 

60  3i 

83.3 

79'  ^ 

•4.0 

6 

Caraccas,  South  America  .... 
Samarang,  Java  

10  3i 
—  6  5o 

67     5 
—  no  3o 

73.5 

84.2 

69.4 
80.  i 

4.i 
4.1 

i 
i 

Bogota,  South  America  

4  36 

?4  1  4 

61  .0 

57.6 

4.3 

i 

Tovar  South  America  

10  3i 

67  3o 

66.0 

61.5 

4.5 

i 

Barbadoes,  West  Indies  

i3     4 

5g  37 

80.6 

76.  i 

4.5 

i 

St.  Bartholomew,  West  Indies  . 
La  Guayra,  South  America.  .  . 
Freetown,  West  Africa  

17  53 
10  37 

8  3o 

62  54 
67     7 
i3   10 

83.3 
81.1 

82.0 

78.7 
76.5 
77.0 

4.6 
4.6 
5.o 

i 
i 
i 

Batavia,  Java  

—  69 

—  106  53 

80.0 

?5  .0 

5.o 

i 

Trevandrum,  Hindostan  

8  3i 

—  74  5o 

82.7 

77-7 

5.o 

8 

Raiatea,  Society  Islands  

—  1  6  4o 

i56   16 

80.9 

75  .  7 

5  .2 

i 

Antigua,  West  Indies  

17     8 

61  48 

81.9 

76.5 

5.4 

i 

Paramaribo,  Dutch  Guiana.  .  . 
Guatemala,  Central  America.  . 
St.  Thomas,  West  Indies  

5  44 
i4  36 
18  21 

55   i3 
90  28 
64  56 

83.4 
71.2 
83.7 

78.0 
65.8 
78.2 

5.4 
5.4 
5.5 

2 
I 
I 

Upper  Park  Camp,  Jamaica  .  . 

17  58 

76  5o 

83.o 

77.5 

5.5 

I 

TABLE  XXI. — PLACES  HAVING  A  GREAT  MONTHLY  RANGE  OP  TEMPERATURE. 


Place. 

Latitude. 

Longitude. 

Hottest 
Month. 

Coldest 
Month. 

Differ- 
ence. 

No.  of 
Years. 

Jakutsk,  Siberia  

O        / 
62       2 

o       ' 
—  129   44 

O 
62.  2 

O 

—43.8 

O 
1  06.0 

17 

Ustjansk,  Siberia  

7O    55 

—  138    24 

52  .  7 

—38.q 

01  .  6 

4 

Fort  Churchill,  Hudson  Bay.  . 
Nertchinsk,  Russia  

5g     2 
5i    18 

93   10 
—  119  20 

58.o 
64.o 

—28.0 

—  21.3 

86.0 
85.3 

I 

i4 

Udskoi  Ostrog,  Siberia  • 

54  3o 

—  i  34  58 

61  .0 

—  21  .6 

82.6 

i 

Utkinsk,  Russia  

57  45 

—    5q    22 

73.9 

-  6.9 

80.8 

i 

Kirgis,  Russia  

5o     o 

—  60     o 

67.5 

—  i3.o 

8o.5 

i 

Uralsk,  Russia  

5r   ii 

—    5l     22 

78.4 

—    O.2 

78.6 

3 

Fort  Simpson,  British  America 
Cumberland  House,  Br.  Amer. 

62  ii 

53  57 

121     32 

IO2     17 

63.5 
61.8 

—  13.5 

—  13.2 

77.0 
75.0 

2 
I 

Melville  Island,  Br.  America.  . 
Fort  Franklin,  Great  Bear  Lake 
Boothia  Felix,  Br.  America.  .  . 
Barnaul,  Russia  

74  47 
65  12 
69  5g 
53  20 

I  10    48 

123   i3 
92     I 
—  83  27 

42.4 

52.  I 

4i.3 

67.5 

—32.5 

—  22.3 
—  32.0 

—  5.6 

74-9 
74-4 
73.3 
73.1 

I 
I 

3 
6 

Msclmey  Tugilsk,  Russia  .... 

57  56 

—  60     8 

70.  3 

I  .2 

69.  i 

2 

Bo^oslowsk,  Russia  

59  45 

—  5q  5q 

66.0 

—   3.i 

69.  i 

6 

Tomsk,  Russia  

56  3o 

—  85   10 

65.3 

—  3.5 

68.8 

5 

Irkutsk,  Russia  

52     17 

—  104  17 

64.8 

—  3.3 

68.1 

10 

Igloolik,  British  America.  .  .  . 
Orenburg,  Russia  

69    21 

5o  46 

81   53 
—  55     6 

Sg.i 

67.2 

—28.2 
0.8 

67.3 
66.4 

i 

8 

272 


TABLES   XXII.,  XXIII. — TEMPERATURE,  ETO. 


TABLE  XXII. — PLACES  HAVING  SMALL  ABSOLUTE  RANGE  OP  TEMPERATURE. 


Place. 

Latitude. 

Longitude. 

Highest. 

Lowest. 

Kanpe. 

Barbadoes,  West  Indies  
Pulo  Penang,  Malacca  Strait  . 
Ourapao,  South  America  .... 
San  Luis  de  Maranha,  Brazil.  . 
Surinam,  Dutch  Guiana  

o     / 
i3     5 
5  25 

12       6 
—    2    3l 

5  38 

O        ' 

59  37 
—  100   19 
69  20 
44  18 
55  20 

O 

86 

90 

91 
92 
90 

O 
72 
76 

75 
76 
70 

O 

i4 

i4 
16 
16 
20 

La  Guayra,  Venezuela  

10  36 

67      7 

91 

7O 

21 

Cayenne,  Guiana  

4  56 

52    11 

87 

65 

22 

Amboyno,  E.  Archipelago  .  .  . 
Tahiti,  South  Pacific  

3  4i 
—  17  20 

—  128  17 
iAo  3o 

91 

OO 

68 
65 

23 
25 

Maracaibo,  Venezuela  

10  43 

71   52 

99 

70 

29 

Singapore,  Malacca  

i    17 

—  io3  5o 

95 

66 

2Q 

Quito,  Equador  

—  o  i4 

78  45 

72 

43 

20 

Lima,  Peru  

—  12        3 

77     8 

86 

57 

20 

St.  Helena,  South  Atlantic  .  .  . 
Port  Louis,  Isle  of  France  .  .  . 
Martinique,  West  Indies  

—  15  55 
—  20   10 

i4  4o 

5  43 
—  57  3o 
61     3 

82 

91 
95 

52 

60 
63 

3o 
3i 

32 

Trinidad,  Caribbean  Sea  .... 
St.  Bartholomew,  West  Indies. 
Paramaribo,  Guiana  

10  3g 
17  54 
5  45 

61  23 
62  54 
55  i3 

93 
97 
94 

61 

64 
61 

32 

33 
33 

Funchal,  Madeira  

32  38 

16  56 

85 

5i 

34 

Vera  Cruz  Mexico  

10     12 

06     o 

06 

61 

35 

Fort  Dundas,  Australia  

V      c 
—  I  I     25 

—  132    25 

IOO 

63 

37 

TABLE  XXIII. — PLACES  HAVING*LARGE  ABSOLUTE  RANGE  OP  TEMPERATURE. 


Place. 

Latitude. 

onpitude. 

Highest. 

Lowest. 

Kllllj.'!'. 

Barnaul,  Asia  

O        / 

53  20 

o        / 

—  83  27 

o 
96 

O 
—  67 

O 

i63 

Jakutzk,  Siberia  

62       2 

—  129  44 

86 

—  76 

162 

Nijnei  Taguilsk,  Ural  Mts.  .  .  . 
Bogoslowsk,  Ural  Mts  

57  56 
5o  45 

—  60     8 
—  5o  5o 

95 

01 

—60 

—63 

i55 

1  54 

Fort  Reliance,  Brit.  America.  . 

62  46 

109     o 

81 

—70 

i5i 

Zlatouste,  Ural  Mts  

55  ii 

—  59  45 

88 

—  57 

1  45 

Nertchinsk,  Siberia  

5i   18 

—  119  20 

94 

—  5o 

1  44 

Catherinenburg,  Ural  Mts.  .  .  . 
Moscow,  Russia  

56  5o 
55  45 

-  60  34 
—  37  34 

94 
94 

-48 
—47 

1  42 

i4i 

Montreal,  Canada  

45  3i 

73    32 

1  02 

—38 

i4o 

Lowville,  New  York  

43  4? 

75  33 

IOO 

—  4o 

i4o 

Quebec,  Canada  

46  49 

71     12 

9Q 

—  4o 

1  39 

Fort  Howard,  Wisconsin  

44  3o 

88     5 

IOO 

—38 

i38 

Nijnei  Kolymsk,  Siberia  

68  32 

—  i  60  56 

72 

—65 

i37 

Enontakis,  Lapland  

68  3o 

—  20  47 

79 

—58 

i37 

Kazan,  Russia  

55  48 

—  4o     7 

07 

—  4o 

i37 

Fort  Snelling,  Minnesota  .... 
Montgomery,  New  York  .... 
Tornea,  Lapland  

44  53 

4l     32 

66  27 

93  10 
74     o 
—  23  55 

IOO 

io4 

77 

-37 
—33 

—58 

i37 
i37 
1  35 

Lougan,  Russia  

48  35 

—  39  21 

IOI 

—33 

1  34 

Granville,  New  York  

44  20 

73  17 

J02 

—  3i 

i33 

St.  Louis,  Missouri  

38  37 

oo   1  5 

I  08 

—  25 

1  33 

Kinderhook,  New  York  

42    22 

73  43 

I  O2 

—  3o 

i32 

Chicago,  Illinois  

4i  53 

87  37 

IO2 

—  3o 

l32 

Albany,  New  York  

42  39 

73  44 

99 

—  32 

i3i 

TABLES  XXIV.,  XXV. — HEIGHT  OF  THE  SNOW  LINE,  ETC.    273 


TABLE    XXIV. 

HEIGHT  OF  THE  SNOW  LINE  ABOVE  THE  SEA. 


Mountains. 

Latitude. 

Height. 

Mountains. 

Latitude. 

Height. 

Spitzbergen  

O      ' 

78       N. 
71  10 
70  3o 
67 
65 
62 

60  55 

5g  3o 
Sg  3o 

56  3o 

5o 
4545 
43  20 

43 

4245 

39  42 

38  33 
37  3oN. 

feet. 
o 

2,4OO 
3,178 

3,835 
3,o8o 
5,i55 

4,470 

5,249 
5,423 

3,5io 

7,246 
8,890 
10,818 
12,467 
8,676 
14,170 
10,705 
9,485 

Bolor  Mountains  .  .  . 
Hindu  Kho  ....... 

O     ' 
3?  3oN. 
34  3o 
3o 

28 

J9 

i3  10 

8    5 

446 
2  i5N. 

0      0 

o4i  S. 
16 
18 
33 
4a  3o 
53  3oS. 

Feet. 
17,010 
l5,735 
17,392 
l4,28o 

i4,668 
:4,o65 

14,920 

i5,325 
i5,38i 

15,960 
15,924 
17,250 
i8,524 
14,708 
6,000 
3,7o7 

North  Cape 

Mountains  of  Norway 
Sulitelma,  Lapland.  . 
Iceland  

Himalaya,  nort 
Himalaya,  sout 
Cordilleras  of 
Mexico 
Mountains  of 
Abyssinia 
Sierra  Nevada 
of  Merida 
Volcano  of  Tol 
Puraci,  S.  Amei 

Nevados  of  Qui 
Cotopaxi  .... 

a  side 
i  side 

ma.  . 
ica.  . 

to.  .  . 

Mountains  of  Norway 
Aldan  Mount-  ) 
ains,  Siberia  J  *  *  *  ' 
Kamtschatka  .  .  .  . 

Mountains  of  Norway 
Unalaschka,  W.  ) 
America        )    '  ' 
Altai  Mountains  .... 
Alps  

Caucasus 

Rocky  Mountains.  .  . 
Pyrenees  

Arequipa,  Bolrv 
Paachata,  Bolrv 
Portillo,  Chili. 
Cordilleras,  Chi 
Magellan  Straii 

ia.  .  . 
ia.  .  . 

Ararat  

Mount  Argseus  

li.  .  . 

Etna  

TABLE   XXV. 

FACTORS  FOB  MULTIPLYING  THE  EXCESS  OF  THE  DRY-BULB  OVER  THE  WET- 
BULB  THERMOMETER,  TO  FIND  THE  EXCESS  OF  THE  TEMPERATURE  OF  THE 
AIR  ABOVE  THAT  OF  THE  DEW-POINT. 


Dry-bulb 
Therm. 

Factor. 

Dry-bulb] 
Therm. 

Factor. 

Dry-bulb 
Therm. 

Factor. 

Dry-bulb 
Therm. 

| 
! 

£ 

Dry-bulb  1 
Therm. 

Factor. 

Dry-bulb 
Therm. 

Factor. 

o 

o 

o 

o 

O 

o 

IO 

8.78 

25 

6.53 

4o 

2.29 

55 

i  .96 

70 

1.77 

85 

i.65 

1  1 

8.78 

26 

6.08 

4i 

2.26 

56 

1.94 

71 

1  .76 

86 

i.65 

12 

8.78 

27 

5.6r 

4a 

2.23 

5? 

i  .92 

72 

i.75 

87 

1.64 

13 

8.77 

28 

5.  12 

43 

2.  2O 

58 

i  .90 

73 

i.?4 

88 

1.64 

14 

8.76 

29 

4.63 

44 

2.l8 

59 

1.89 

74 

i.73 

89 

1.63 

l5 

8.75 

3o 

4.i5 

45 

2.  l6 

60 

1.88 

75 

i  .72 

9° 

i.63 

16 

8.70 

3i 

3.70 

46 

2.  l4 

61 

1.87 

76 

1.71 

91 

i  .62 

*7 

8.62 

32 

3.32 

47 

2.  12 

62 

1.86 

77 

i  .70 

92 

i  .62 

18 

8.5o 

33 

3.oi 

48 

2.  10 

63 

i.85 

78 

i  .69 

93 

1.61 

*9 

8.34 

34 

2.77 

49 

2.08 

64 

i.83 

79 

i  .69 

94 

i  .60 

20 

8.i4 

35 

2.60 

5o 

2.06 

65 

1.82 

80 

1.68 

95 

i  .60 

21 

7.88 

36 

2.5o 

5i 

2.04 

66 

1.81 

81 

1.68 

96 

i  .5g 

22 

7.60 

3? 

2.42 

52 

2.  O2 

67 

i.  80 

82 

i  .67 

97 

i  .  5g 

23 

7.28 

38 

2.36 

53 

2.OO 

68 

1.79 

83 

i  .67 

98 

i.58 

24 

6.92 

39 

2.32 

54 

1.98 

69 

1.78 

84 

1.66 

99 

i.58 

274        TABLE   XXVI. — RELATIVE   HUMIDITY   OF  THE   AIR. 


TABLE   XXVI. 

RELATIVE  HUMIDITY   OF  THE  AIR. 


Tempi 
of  Air. 

Difference  of  Temperature  of  the  Air  and  of  the  Dew  Point. 

0° 

1° 

2° 

3° 

4° 

83 

83 
83 
83 
83 

5° 

6° 

7° 

8° 

9°  |10°  12° 

14°  16°  18° 

20°  22°  24° 

6° 
7 

8 
9 

10 

too 

IOO 
IOO 
IOO 
IOO 

96 
96 
96 
96 
96 

91 
91 
91 
91 
91 

87 
87 
87 
87 
87 

80 
80 
80 
80 
80 

76 
76 
76 
76 
76 

72 
73 
73 
73 
73 

69 
69 
69 
69 
70 

66 
66 
66 
66 
66 

63 
63 
63 
63 
63 

57 
5? 
5? 

58 
58 

52 
52 
52 
52 
52 

47 
47 
47 
48 
48 

43 
43 
43 
43 
43 

39 

39 
39 
39 
39 

35 
35 
35 
35 
36 

32 
32 
32 
32 
32 

ii 

12 

i3 

i4 
i5 

IOO 
IOO 
IOO 
IOO 
IOO 

96 

95 

95 
95 
95 

91 
91 
91 
91 
91 

87 
87 
87 
87 
87 

83 
83 
83 
83 
83 

80 
80 
80 
80 
80 
80 
80 
80 
80 
80 

76 
76 
76 
76 
76 

73 
?3 
?3 
73 
73 

70 
70 
7° 
7° 
?o 
70 

7° 
69 
69 
69 

66 
66 
66 
66 
67 
66 
66 
66 
66 
66 

63 
63 
63 

64 
64 

58 
58 
58 
58 
58 

53 
53 
53 
53 
53 

4» 
48 
48 
48 
48 

43 
44 
44 
44 
44 

44 
44 
44 
44 

44 

39 
4o 
4o 
4o 
4o 

36 
36 
36 
36 
36 

33 
33 
33 
33 
33 

16 

i? 

18 

19 

20 

IOO 
IOO 
IOO 
IOO 
IOO 

95 
95 
95 
95 

96 

9' 
91 
91 
91 
91 

87 
87 
87 
87 
87 

83 
83 
83 
83 
83 

76 
76 
76 
76 
76 

73 
73 
73 
?3 
73 

64 
64 
63 
63 
63 

58 
58 
58 
58 
58 

53 
53 
53 
53 
53 

48 
48 
48 
48 
48 

4o 
4o 
4o 
4o 

4o 

36 
36 
36 
37 

3? 

33 
33 
33 
33 
33 

21 
22 
23 
24 
25 

IOO 
IOO 
IOO 
IOO 
IOO 

96 
96 
96 
96 
96 

91 
91 
91 
91 
91 

87 
87 
87 
87 

88 

83 
83 
83 
84 

84 

80 
80 
80 
80 
80 

76 
76 
76 
76 
76 

73 
?3 
?3 
73 
73 

69 
70 
70 

7° 
70 

66 
66 
66 
67 
67 

63 
63 
64 
64 
64 
64 
64 
64 
64 
65 

58 
58 
58 
58 
58 

53 
53 
53 
53 

53 

48 
48 
48 
48 
48 

44 
44 
44 
44 
44 

4o 
4o 
4o 
4o 
4o 

3? 
37 
3? 
37 
3? 

33 
33 

34 
34 
34 

26 
27 
28 
29 

3o 

IOO 
IOO 
IOO 
IOO 
IOO 

96 
96 
96 
96 
96 

92 
92 
92 
92 
92 

88 
88 
88 
88 
88 

84 
84 
84 
84 
84 

«o 
80 
80 
81 
81 

77 
77 
77 
77 

77 

77 
78 
78 
78 
78 

73 
73 
74 
74 
74 

7° 
7° 
7° 
70 

71 
?i 
7i 
?i 
72 
72 

67 
67 
67 
67 

68 

58 
58 
59 
59 
59 

53 
53 
53 

54 
54 

49 
49 
49 
49 
49 

44 
44 
45 
45 
45 

4o 
4i 
4i 
4i 
4i 

37 
37 
37 
3? 

3? 

34 
34 
34 
34 

34 

3i 

32 

33 

34 
35 

IOO 
IOO 
IOO 
IOO 
IOO 

96 
96 
96 
96 
96 

92 
92 
92 
92 
92 

88 
88 
89 
89 
89 

84 
85 
85 
85 
85 

81 
81 
81 
82 
82 

74 
74 
75 
75 
75 

68 
68 
68 
69 
69 

65 
65 
65 
66 
66 

59 
60 
60 
60 
60 

54 
54 
55 
55 
55 

49 
5o 
5o 
5o 
5i 

45 
45 
46 
46 
46 

4i 
4i 

42 
42 

42 

37 
38 
38 
38 
38 

34 
34 
35 
35 
35 

36 

3? 
38 
39 

4o 

IOO 
IOO 
IOO 
IOO 
IOO 

96 
96 
96 
96 
96 

92 
92 
92 
92 
92 

89 
89 
89 

By 
89 

85 
85 
85 
85 
86 

82 
82 
82 
82 
82 

79 
79 
79 
79 
79 

75 
76 
76 
76 
76 

72 
72 
73 
73 
73 

69 
69 

7° 
70 

7° 

66 
67 
67 
67 
67 

61 
61 
61 
62 
62 

56 
56 
56 
56 
57 

5i 
5i 
5i 

52 

52 

46 
47 
47 
47 

48 

42 

43 
43 
43 
43 

39 
39 
39 
39 

4o 

35 
36 
36 
36 
36 

4i 

42 

43 

44 
45 

IOO 
IOO 
IOO 
IOO 
IOO 

96 
96 
96 
96 
96 

93 
93 
93 
93 
93 

89 
89 
89 
89 
89 

86 
86 
86 
86 
86 

82 
82 
82 
83 
83 

79 
79 
79 
79 
80 

76 
76 
76 
76 
76 

73 
73 
73 
73 
74 

70 
7° 
7i 
71 
71 

67 
68 
68 
68 
68 

62 
62 
62 
63 
63 

57 
57 
58 
58 
58 

52 

53 
53 
53 
53 

48 
48 
48 
49 
49 

44 
44 
44 
45 
45 

4o 
4o 
4i 
4i 
4i 

36 
37 
3? 
37 
38 

46 

4? 
48 

49 
5o 

IOO 
IOO 
IOO 
IOO 
IOO 

96 
96 
96 
96 
96 

93 
93 
93 
93 
93 

89 
89 
89 
89 
89 

86 
86 
86 
86 
86 

83 
83 
83 
83 
83 

80 
80 
80 
80 
80 

77 
77 
77 
77 
77 

74 
74 
74 
74 
74 

7i 
7i 
71 
?i 
7i 

68 
68 
68 
68 
69 

63 
63 
63 
63 
63 

58 
58 
58 
59 
59 

54 
54 
54 
54 
54 

49 
49 
5o 
5o 
5o 

45 
45 
46 
46 
& 

4i 

42 
42 
42 

& 

38 
38 
38 

39 
39 

TABLE   XXVI. — RELATIVE   HUMIDITY   OF   THE   AIR.         275 


TABLE   XXVI. 

RELATIVE   HUMIDITY   OP   THE   ATR. 


Temp, 
of  Air. 

Difference  of  Temperature  of  the  Air  and  of  the  Dew  Point. 

0°  1° 

2° 

3° 

4°  5° 

6° 

7° 

8° 

9° 

10°  12°  14°|16°  18° 

20° 

22° 

24° 

5l° 

52 

53 

54 
55 

100  96 
ioo  96 
100  |  96 
100  i  96 
zoo  96 

93 
93 
93 
93 
93 

89 
89 
9° 
9° 
9° 

86 
86 
86 
86 
86 

83 
83 
83 
83 
83 

80 
80 
80 
80 
80 

77 
77 
77 
77 
77 

74 
74 
74 
74 
74 

71 
71 
72 
72 
72 

69 
69 
69 
69 
69 

64 
64 
64 
64 
64 

59 
59 
59 
59 
59 

54 
55 
55 
55 
55 

5o 
5o 
5i 
5i 

5i 

46 
47 
4? 
4? 

47 

43 
43 
43 
43 
43 

39 
39 
4o 
4o 
4o 

56 

57 
58 
59 
60 

IOO 
100 
IOO 
IOO 
IOO 

96 
96 
96 
96 
96 

93 
93 
93 
93 
93 

90 

9° 
9° 
9° 
9° 

Sti 
86 
87 
87 
87 

83 
83 
83 

84 
84 

80 
80 
80 

81 
81 

77 
77 

78 
78 

78 

75 

75 

75 
75 
75 

72 
72 
72 
72 

72 

69 
69 
69 
70 

70 

64 
64 
64 
65 
65 

59 
60 
60 
60 
60 

55 
55 
55 
55 
56 

5i 
5i 
5i 
5i 

52 

47 
47 
47 
48 
48 

44 
44 
44 
44 

44 

4o 
4o 
4i 

4i 
4i 

61 

62 
63 
64 
65 

IOO 
IOO 
IOO 

IOO 
IOO 

96 

97 
97 
97 

97 

93 
93 
93 
93 
93 

9° 
9° 
9° 
9° 
9° 

87 
87 
87 
87 

87 

84 
84 
84 
84 

84 

81 
81 
81 
81 
81 

78 
78 
78 
78 
78 

75 

75 
75 
75 

75 

72 
72 
72 
?3 
73 

7° 
7° 
70 
70 

70 

65 
65 
65 
65 
65 

60 
60 
60 
60 
61 

56 
56 
56 
56 
56 

52 
52 
52 
52 
52 

48 
48 
48 
48 
49 

44 
44 
45 
45 
45 

4i 
4i 
4i 
4i 

42 

66 
67 
68 
69 

70 

IOO 
IOO 
IOO 
IOO 
IOO 

97 
97 
97 
97 
97 

93 
93 
93 
93 
93 

9° 
9° 

9° 
90 
90 

»7 

87 
87 
87 
87 

84 
84 
84 
84 
84 

81 
81 
81 
81 

81 

78 
78 
78 
78 
78 

75 
76 
76 
76 
76 

73 
?3 
73 
?3 
73 

70 
7° 
7° 
7* 

71 

65 
65 
66 
66 
66 

6! 

61 
61 
61 

61 

56 
57 
5? 
5? 

57 

52 

53 
53 
53 

53 

49 
49 
49 
49 
49 

45 
45 
45 
46 
46 

42 
42 
42 
42 
42 

?i 
72 

73 

74 
75 

IOO 
IOO 
IOO 
IOO 
IOO 

97 
97 
97 
97 
97 

97 
97 
97 
97 
97 

93 
93 
93 
93 
93 

9° 
90 

90 
9° 
9° 

87 
87 
87 
87 

87 

«7 
87 
88 
88 
88 

84 
84 
84 
84 
84 

81 
81 
81 
82 
82 
"82" 
82 
82 
82 
82 

79 
79 
79 
79 

79 

79 
79 
79 
79 
79 

76 
76 
76 
76 

76 

76 
76 
76 
77 

77 

?3 
73 
73 

74 
74 

74 
74 
74 
74 
74 

71 
?i 
7i 
?i 
7i 

66 
66 
66 
66 
66 
~66~ 
67 
67 
67 
67 

61 
61 
62 
62 
62 
~6z 
62 
62 
62 
62 

57 
5? 
5? 
5? 
58 

"58" 
58 
58 
58 
58 

53 
53 
53 
53 

54 
54 
54 
54 
54 
54 

49 
49 
5o 
5o 
5o 

46 
46 
46 
46 
46 

43 
43 
43 
43 
43 

76 

77 

78 

79 
80 

IOO 
IOO 
IOO 
IOO 
IOO 

94 
94 
94 
94 
94 

9° 
90 
90 
9i 
91 

85 
85 
85 
85 
85 

7i 
7» 
?i 

7i 
72 

5o 
5o 
5o 
5o 
5i 

47 
47 
4? 
47 
47 

43 
43 
44 
44 
44 

81 

82 
83 
84 
85 

IOO 
IOO 
IOO 
IOO 
IOO 

97 
97 
97 
97 
97 

94 
94 
94 
94 
94 

91 
91 
91 
9* 
91 

88 
88 
88 
88 
88 

85 
85 
85 
85 
85 

82 
82 
82 
82 
82 

79 
79 
79 
79 
80 

77 
77 
77 
77 

77 

74 
74 
74 
74 
75 

72 
72 
72 
72 

72 

67 
67 
67 
67 
67 

63 
63 
63 
63 
63 

58 
58 
59 
59 
59 

54 
55 
55 
55 
55 

5i 
5i 
5i 
5i 
5i 

47 
4? 
48 
48 

48 

44 
44 
44 
44 
45 

86 

8? 
88 
89 
90 

IOO 
IOO 
IOO 
IOO 
IOO 

97 
97 
97 
97 
97 

94 
94 
94 
94 
94 

91 
91 

91 
91 

91 

88 
88 
88 
88 
88 

85 
85 
85 
85 
85 
"85" 
85 
85 
86 
86 

82 
82 
82 
83 
83 

80 
80 
80 
80 
80 

77 
77 
77 
77 

77 

75 
75 
75 
75 
75 

72 
72 
72 
72 
73 

67 
68 
68 
68 
68 

63 
63 
63 
63 

64 

59 
59 

59 
59 

59 

55 
55 
55 
55 
56 

5i 

52 
52 
52 

52 

48 
48 
48 
48 
49 

45 
45 
45 
45 

45 

91 
92 
93 

94 
1  95 

IOO 

IOO 
IOO 
IOO 

IOO 

97 
97 
97 

97 
97 

94 
94 
94 
94 
94 

91 
91 
91 
91 
91 

88 
88 
88 
88 
88 

83 
83 
83 
83 
83 

80 
80 
80 
80 
80 

77 
78 
78 
78 
78 

75 
75 
75 
75 

75 

73 
73 
73 
73 
73 

68 
68 
68 
68 
68 

64 
64 
64 
64 
64 

60 
60 
60 
60 
60 

56 
56 
56 
56 
56 

52 
52 
52 
52 

53 

49 
49 
49 
49 
49 

45 
46 
46 
46 
46, 

276      TABLE  XXVII. — ELASTIC   FORCE   OF  AQUEOUS   VAPOR. 


TABLE   XXVII. 

ELASTIC   FORCE   OP   AQUEOUS   VAPOR. 


Tempera- 
ture 

Pure*  of 
Vapor. 

Tempera- 
ture. 

Korc.  of 
Vapor. 

lempera- 
ture. 

Force  of 
Vapor. 

Tempera- 
ture. 

Force  of 

Vapor. 

Tempera- 
ture. 

Force  of    ] 
Vapor,      j 

O 

loch. 

O 

Inch. 

O 

Inch. 

O 

Inch. 

O 

Inch. 

—  3o 

.009 

4? 

.323 

69 

.708 

8l.4 

.071 

91.4 

.4?3 

-25 

.012 

47-5 

.329 

69.3 

.716 

8l.6 

.078 

91  .6 

.482 

—  20 

.Ol6 

48 

.335 

69.7 

.725 

8l.8 

.o85 

91.8 

.4gi 

-i5 

.O2I 

48.5 

.34i 

70 

.733 

82.0 

.092 

92.0 

.5oi 

—  IO 

.027 

49 

.348 

70.3 

•  74o 

82.2 

.099 

92.2 

.5io 

—  5 

.034 

49-5 

.354 

70.7 

.75i 

82.4 

.  1  06 

92.4 

.520 

o 

.o43 

5o 

.36i 

71 

.758 

82.6 

.114 

92.6 

.529 

+    2 

.o48 

5o.5 

.367 

71.3 

.766 

82.8 

-  121 

92.8 

.539 

4 

.052 

5i 

•  374 

71.7 

.776 

83.o 

.128 

93.0 

.548 

6 

.o57 

5i.5 

.38i 

72 

.784 

83.2 

.i35 

93.2 

.558 

8      ; 

.062 

52 

.388 

72.3 

.792 

83.4 

.i43 

93.4 

.568 

IO 

.068 

52.5 

.395 

72.7 

.8o3 

83.6 

.  i5o 

93.6 

•577 

12 

.075 

53 

.4o3 

73 

.811 

83.8 

.i58 

93.8 

.587 

i4 

.082 

53.5 

.4io 

73.3 

.820 

84.o 

.i65 

94.0 

.597 

16 

.090 

54 

.4i8 

73.7 

.83i 

84-2 

.i73 

94.2 

.607 

id 

.098 

54.5 

.425 

74 

.83g 

84-4 

.180 

94-4 

.617 

20 

.108 

55 

.433 

74.3 

.848 

84-6 

.188 

94.6 

.627 

21 

.n3 

55.5 

•  44i 

?4-7 

.859 

84.8 

.  195 

94.8 

.637 

22 

.118 

56 

.449 

75.0 

.868 

85.o 

.203 

gS.o 

.64? 

23 

.123 

56.5 

•  457 

75.2 

.873 

85.2 

.211 

95.2 

.657 

24 

.  129 

5? 

.465 

75.4 

.879 

85.4 

.219 

95.4 

.667 

25 

.i35 

57.5 

.4?4 

75.6 

.885 

85.6 

.226 

95.6 

.677 

26 

.i4i 

58 

.482 

75.8 

.891 

85.8 

.234 

95.8 

.688 

27 

•  i47 

58.5 

.491 

76.0 

.897 

86.0 

.242 

96.0 

.698 

28 

.i53 

59 

.5oo 

76.2 

.903 

86.2 

.250 

96.2 

.708 

29 

.160 

59.5 

.Sog 

76.4 

.909 

86.4 

.258 

96.4 

.719 

3o 

.  167 

60 

.5i8 

76.6 

.giS 

86.6 

.266 

96.6 

.729 

3i 

.174 

6o.5 

.527 

76.8 

.921 

86.8 

.274 

96.8 

•  ?4o 

32 

.18: 

61 

.536 

77.0 

.927 

87.0 

.282 

97.0 

.75i 

33 

.188 

6i.5 

.546 

77.2 

.933 

87.2 

.290 

97.2 

.761 

34 

.  196 

62 

.556 

77-4 

•939 

87-4 

.298 

97-4 

.772 

35 

.2O4 

62.5 

.566 

77.6 

•  946 

87.6 

.307 

97.6 

.783 

36 

.212 

63 

.576 

77.8 

.952 

87.8 

.3i5 

97.8 

•794 

37 

.220 

63.3 

.582 

78.0 

.958 

88.0 

.323 

98.0 

.8o5 

38 

.229 

63.7 

.Sgo 

78.2 

.964 

88.2 

.332 

98.2 

.816 

39 

.238 

64 

.596 

78.4 

.971 

88.4 

.34o 

98.4 

.827 

4o 

.248 

64.3 

.602 

78.6 

•977 

88.6 

.349 

98.6 

.838 

4o.5 

.252 

64-7 

.611 

78.8 

.984 

88.8 

.357 

98.8 

.849 

4i 

.257 

65 

.617 

79.0 

.990 

89.0 

.366 

99.0 

.861 

4i.5 

.262 

65.3 

.624 

79.2 

•997 

89.2 

.375 

99.2 

.872 

42 

.267 

65.7 

.632 

79-4 

i  .oo3 

89.4 

.383 

99.4 

.883 

42.5 

.272 

66 

.639 

79.6 

i  .010 

89.6 

.392 

99.6 

.895 

43 

.277 

66.3 

.646 

79-8 

i  .016 

89.8 

.4oi 

99.8 

.906 

43.5 

.283 

66.7 

.655 

80.0 

I  .023 

90.0 

.4io 

IOO.O 

.918 

44 

.288 

67 

.662 

80.2 

i  .o3o 

90.2 

.419 

IOO.2 

.929 

44.5 

.294 

67.3 

.668 

8o.4 

1.037 

90.4 

.427 

I  OO.4 

•  94i 

45 

.299 

67.7 

.678 

80.6 

i.o43 

90.6 

.436 

100.6 

.953 

45.5 

.3o5 

68 

.685 

80.8 

i  .o5o 

90.8 

.446 

100.8 

.965  ! 

46 

.3n 

68.3 

.692 

81.0 

i  .057 

91  .0 

.455 

IOI  .O 

•977 

46.5 

.3i7 

68.7 

.701 

81.2 

i  .o64 

91  .2 

.464 

IOI  .2 

.988 

TABLE   XXVIII. — PRESSURE  AND  VELOCITY  OF   THE  WIND.    277 


TABLE   XXVIIL 

FOR  COMPARING  THE  PRESSURE  AKD  VELOCITY  OF  THE  WIND. 


Pressure, 
01.  u«r 
iq     foot. 

Velocity, 
miles 
per  hour. 

Pressure, 
Ibs.  p«r 

sq.  loot. 

Velocity, 

nii.es 
per  hour. 

Pressure, 
Ibs.  per 

!  sq.  t..ot. 

Velocity, 
nil.es 
per  hour. 

Pressure, 
Ibs.  per 
eq.  fout. 

Velocity, 
,,e"r'hour. 

Pressure, 
Ibs.  per 
sq.  foot. 

Velooty, 
miles 
per  hour. 

0.0» 

1  .OOO 

6.75 

36.742 

iiy.OO 

5g.  160 

28.25 

,i>.  166 

39.25 

88.600 

O.25 

1.767 

7.OO 

37.4i6 

17.75 

59.58i 

28.  5o 

75.498 

Sg.So 

88.881 

o.5o 

2.5oo 

7.25 

38.  078 

iS.OO 

60.000 

28.75 

75.828 

39.75 

89.162 

0.76 

3.061 

7.5o 

38.729 

18.25 

6o.4i5 

29.00 

76.  157 

4o.oo 

89.442 

i  .00 

3.535 

7.75 

39.370 

i8.5o 

60.827 

29.25 

76.  485 

40.26 

89.721 

2 

5.  ooo 

8.00 

4o.ooo 

i8.75 

61.237 

29.50 

76.811 

4o.5o 

9O.OOO 

3 

6.123 

8.25 

40.620 

19.  oo 

61.644 

29.75 

77-i36 

4o.75 

90.277 

4 

7.071 

8.5o 

4l  .201 

19.25 

62.048 

3o.oo 

77-459 

4  1  .00 

90.553 

5 

7.905 

8.75 

4i.833 

19.5© 

62.449 

3o.25 

77.781 

4i.25 

90.829 

6 

8.660 

9.00 

42.426 

I9.75 

62.849 

3o.5o 

78.102 

4i  .5o 

91  .  io4 

7 

9.354 

9.25 

43.0H 

20.00 

63.245 

30.75 

78.421 

4i.75 

91.378 

8 

IO.OOO 

g.So 

43.588 

20.25 

63.639 

3  1  .00 

78.740 

42.00 

91  .65i 

9 

10.606 

9.75 

44-i58 

20.  5o 

64.o3i 

3i.25 

7g.o56 

42.25 

91  .923 

10 

11.180 

IO.OO 

44.721 

20.75 

64.420 

3i  .5o 

79.372 

42.  5o 

92.195 

ii 

II  .726 

10.25 

45.276 

21  .OO 

64.807 

3i.75 

79.686 

42.75 

92.466 

12 

12.247 

io.5o 

45.825 

21  .25 

65.  192 

32.  OO 

80.000 

43.00 

92.736 

i3 

12.747 

10.75 

46.368 

21  .50 

65.574 

32.25 

8o.3n 

43.25 

gS.ooS 

i4 

13.228 

II  .00 

46.904 

21.75 

65.  954 

32.  5o 

80.622 

43.  5o 

93.273 

i5 

13.693 

II  .25 

47-434 

22.OO 

66.332 

32.  75 

80.932 

43.  75 

93.54i 

Pounds. 

ii  .5o 

47-958 

22    25 

66.708 

33.OO 

81  .240 

44.oo 

93.808 

I 

i4.  i4- 

ii  .75 

48.476 

22.  5o 

67.082 

33.25 

81.547 

44.25 

94.074 

1.25 

i5.8n 

I2.0O 

48.989 

22.75 

67.453 

33.  5o 

8i.853 

44.  5o 

94.339 

i.5o 

17.320 

12.25 

49.497 

23.  OO 

67.823 

33.  75 

82.i58 

44.  75 

94.604 

i.75 

18.708 

12.  5o 

So.ooo 

23.25 

68.  190 

34.oo 

82.462 

45.oo 

94.868 

2.OO 

20.000 

12.  75 

50.497 

23.  5o 

68.556 

34.25 

82.764 

45.25 

95.  i3i 

2.25 

21  .  2l3 

i3.oo 

50.990 

23.  75 

68.920 

34.  5o 

83-o66 

45.  5o 

95.393 

2.5o 

22.360 

i3.25 

51.478 

24.00 

69.282 

34.75 

83.366 

45.  75 

95.655 

2.76 

23.452 

i3.5o 

5  i  .961 

24.25 

69.641 

35.oo 

83.666 

46.oo 

95.916 

3.oo 

24.494 

i3.75 

52.44o 

24.  5o 

70.000 

35.25 

83.  964 

46.25 

96.  176 

3.25 

25.4g5 

i4-oo 

52.gi5 

24.75 

70.  356 

35.  5o 

84.261 

46.  5o 

96.436 

3.5o 

26.457 

i4.25 

53.385 

25.00 

70.710 

35.  75 

84.557 

46.  75 

96.695 

3.75 

27.  386 

i4-5o 

53.85i 

25.25 

71  .o63 

36.oo 

84.852 

47.00 

96.953 

4.oo 

28.284 

i4.?5 

54.3i3 

25.  5o 

7i.4i4 

36.25 

85.i46 

47-25 

97.211 

4.25 

29.154 

iS.oo 

54.772 

25.  75 

7i.763 

36.  5o 

85.44o 

47.5o 

97.467 

4.5o 

3o.ooo 

i5.25 

55.226 

26.00 

72.III 

36.75 

85.732 

47.75 

97.724 

4.?5 

3O.822 

i5.5o 

55.677 

26.25 

72.456 

37.00 

86.023 

48.oo 

97-979 

5.oo 

3i.622 

i5.75 

56.124 

26.  5o 

72.801 

37.25 

86.3i3 

48.25 

98.234 

5.25 

32.4o3 

16.00 

56.568 

26.75 

73.i43 

37.5o 

86.602 

48.  5o 

98.488 

5.5o 

33.166 

i6.25 

57.008 

27.00 

73.484 

37.75 

86.890 

48.  75 

98.742 

5.75 

33.911 

i6.5o 

57.445 

27.25 

73.824 

38.oo 

87.177 

49.00 

98.994 

6.00 

34.64i 

16.75 

57.879 

27.  5o 

74.161 

38.25 

87.464 

49.25 

99.247 

6.25 

35.355 

17.00 

58.309 

27.75 

74.498 

38.  5o 

87.749 

49-5o 

99.498 

6.5o 

36.o55 

17.25 

58.736 

28.00 

74.833 

38.75 

88.o34 

49.75 

99.749 

6.75 

36.742 

i7-5o 

Sg.  160 

28.25 

75.166 

39.00 

88.3i7 

So.oo 

JOO.f'OO 

278 


TABLE   XXIX. — AVERAGE   AMOUNT   OF   RAIN 


TABLE    XXIX. 

AVERAGE   ;1MOUNT  OP  RAIN  FOR   EACH  MONTH,  SEASON,  AND  THE  YEAR. 


Station. 

Lat. 

Long. 

Alt. 

Jan. 

Feb. 

March 

April. 

May. 

Paramaribo,  Dutch  Guiana  .  . 
Caraccas,  Venezuela  

O      ' 

5  44 

IO    22 

O        f 

55  i3 

6?     12 

Feet. 

Inches. 

18.74 
I  .00 

Inches. 
I6.5/ 
O    25 

Inches. 
20.75 
I     IO 

Inches. 
21  .  10 

Inches. 

o     o  f 

Matouba,  Guadeloupe  

16   10 

61   5o 

21  .30 

21   38 

18   ii 

Vera  Cruz,  Mexico  

10     12 

06     o 

5o 

5  .  i  o 

3i    /n 

Havana,  Cuba  

23       9 

82  23 

5o 

4.  O7 

3  08 

4  08 

2    28 

Key  West,  Florida  

24    32 

8  1  4S 

IO 

2  .  2O 

2   83 

I     34 

3     02 

Corpus  Christi,  Texas  

27    4? 

Q7    27 

20 

3.06 

2     37 

I     25 

j-9<s 
4  68 

Fort  Brooke,  Texas  

28     o 

82    28 

20 

2  .  2O 

*.  jy 

3  37 

i   o5 

3  24 

St.  Augustine,  Florida  

20  48 

81   35 

25 

2  .  09 

i   63 

2     34 

i   56 

New  Orleans,  Louisiana  

29  67 

90     o 

10 

5.6i 

2.90 

3.90 

3.  29 

4.io 

tfobile,  Alabama  
Savannah,  Georgia  

3o  42 
32     6 

88     i 
81     5 

3o 
3o 

8.89 

2  .  76 

5.07 
2  53 

5.86 
3  69 

4.95 

3.43 

"5    on 

San  Diego,  California  

32    42 

117   i3 

1  5o 

o  83 

n    1n 

Charleston,  South  Carolina.  .  . 
Santa  Fe,  New  Mexico.  ..... 

32  46 
35  4i 

79  56 
1  06     i 

3o 
6846 

2.33 
o.  3i 

3.39 
0.57 

3  .02 
i  .29 

0.77 
i  .72 
0.80 

O.D7 
3.66 

0.74 

Nashville,  Tennessee  
Norfolk,  Virginia  

36     9 
36  So 

86  49 

533 
8 

5.oi 
3  26 

S.gb 

4.91 
3  33 

5.20 
2    80 

4.94 
Q     C./. 

Fort  Massachusetts,  New  Mex. 
San  Francisco,  California  .... 
Sacramento,  California  

37   32 

3?  48 
38  35 

io5  zri 

122    27 
121     28 

8365 
i5o 
5o 

0.23 

3.23 

2    98 

A  .  /4 
0.72 

3.3i 
2   36 

0.94 
4.6i 

0.42 

.3.72 

T      /  /< 

2.l4 

o.48 

r>     ftn 

3.97 

St.  Louis,  Missouri  

38  3? 

GO    1  5 

48i 

2  .  o3 

3  4o 

3    n'-! 

/      nrr 

Washington,  D.  C  

38  53 

77        O 

78 

4  45 

2     75 

0.90 

/     nQ 

4-97 
•5     Cfi 

Cincinnati,  Ohio  

39     6 

84  3o 

55o 

3  35 

3  5i 

<j.  jy 

3    nT 

3    fifi 

/     11 

Philadelphia,  Pennsylvania  .  . 
Pittsburg,  Pennsylvania  

39  57 
4o  32 

75   10 

80       2 

3o 

704 

3.09 

2.18 

2.9^ 

3.43 

2  .  70 

3.64 
3.io 

4.  DO 
3.90 

3.58 

New  York  City,  New  York  .  . 
Salt  Lake  City,  Utah  

4o  43 
4o  46 

74     o 

112       6 

23 

435i 

2.78 

I     23 

2.92 

3.44 
*>  "\f 

3.33 

T       fifi 

4.78 

T           ^    /: 

New  Haven,  Connecticut  .... 
Fort  Laramie,  Dacotah  

4i   18 

42     1  2 

72  55 

io4  48 

60 
45io 

3.53 

i  .99 
3.97 

3.49 

T     "\n 

3.3i 

T     n^ 

4.23 

C      0- 

Detroit,  Michigan  

42    20 

83     2 

58o 

u.  ^7 

2.18 

o.  71 

1.38 

I  .  37 
2.86 

2  .92 

2.73 

Boston,  Massachusetts  
Albany,  New  York  

42     21 

42  4o 

71      3 
73  /5 

71 
i3o 

2.39 

3.i9 

•>     fin 

3.4? 

o     £o 

3.64 

•3      -„ 

3.74 

3Qs 

Fort  Orford,  Oregon  

42  44 

5o 

*  -77 

8  81 

fi     <?1 

80/ 

5fi/ 

Milwaukee,  Wisconsin  

43     4 

87  54 

5g3 

i   3c- 

o  80 

Rochester,  New  York  

43     8 

77  5i 

5o6 

1.88 

i  .4o 

1.  81 

I  .97 

3.o4 

Toronto,  Canada  
Fort  Snelling,  Minnesota  .... 
Wolfville,  Nova  Scotia.  .  . 

43  39 
44  53 
45    6 

79  23 
93   10 
64  25 

34i 
820 

i  .  70 
0.73 

3     27 

i  .09 

O.52 
r,     K./. 

1.61 
i.3o 

1     n/. 

2.57 
2.  l4 

2.98 
3.i7 

1     9^ 

Montreal,  Canada  . 

45  3i 

73  33 

o.iy 
3  •>! 

•3      Q0 

0        /S 

0          On 

Astoria,  Oregon.  . 

46  n 

23  48 

5o 

27.00 

lO.gS 

6.  10 

2.40 
4.38 

S.qSj 

Fort  Brady,  Michigan  . 
Steilacoom,  Washington  Ter.  . 
Fort  Kent,  Maine  

46  39 
47   10 

47     l5 

84  43 

22    25 

68  35 

600 
3oo 

R_R 

i.84 
9-54 

^      rt1} 

i.i3 
5.i6 

i.37 
4.56 

1.83 
4-77 

2.24) 

i.86l 

St.  Johns,  Newfoundland  .... 
Sitka,  Aliashka  

47  33 
57     3 

52    28 

35   18 

D7D 
i4o 
20 

4-74 
7.80 

2.75 
7.32 

1.77 
4.8o 
6.  20 

3.76 
6.83 

2.63 
4.i3 

5.2Q 

FOE  EACH  MONTH,  SEASON,  AND  THE  YEAR. 


279 


TABLE  XXIX. 

AVERAGE  AMOUNT  OF  RATS  FOR  EACH  MONTH,  SEASON,  AND  THE  TEAR. 


Juno. 

July. 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Spring. 

Summer. 

Autumn. 

Winter. 

Year. 

Inchw. 
16.34 
1  6.  oo 
39.53 

21  .20 
25.28 

lacues. 
S.Sg 

i4-o4 
27.95 
59.yo 
5.93 

Inches. 
1.77 

•21  .  l4 
JO.  20 
35.90 
8.90 

laches. 
o.63 
39.37 
i3.i5 
38.90 
:i.i4 

Inches. 
1.46 
i3.4o 
33.ii 
8.00 
1  1  .01 

Inches. 
2.99 

26.80 
24.13 

4.5o 

4-74 

Inches. 
l3.o3 
4.07 
43.07 
o.4o 
i.83 

Inches. 
65.o8 
19.  3o 
62.  1  3 
3  1  .90 
16.47 

Inches. 
24.00 

5i.i8 

77.68 
116.80 
38.ii 

Inches. 
5.08 
79.57 
70.39 

5i  ,4o 
26.89 

Inches. 
48.3i 
5.32 
82.i3 
5.5o 
9.88 

Inches. 

142.47 

i55.37 
292.33 
i83.2o 
91.35 

5.4* 
5.63 
7-o4 
4.27 
4-97 

2.97 
4.89 

II  .  IO 

3.24 

6.66 

4.33 
2.91 

IO.  JO 

3.o3 

5.65 

6.12 

6.73 
6.23 
5.85 

2.20 

4-94 
2.37 

2.40 
2.42 
2.74 

1.77 
i  .o5 

2.OO 

1  .29 

4.68 

2.09 
i  .26 
2.83 
2.08 
4.20 

8.09 
9.94 
8.56 
5.90 
1  1  .29 

12.  78 
i3.43 
28.24 
10.54 

17.28 

12.83 
10.  i5 
IO.63 
9.  56 
9.62 

5.5i 
7.59 
8.o4 
5.8o 
12.71 

^9.21 
_i  .  n 
55.47 
3i.8o 
So.go 

5.OD 

4-84 
o.i5 
5.oo 

1.32 

4.36 
7.57 

O.OI 

6.i5 

4.i8 

8.59 
8.32 
o.3g 
7.53 

3.4o 

4.68 
4.26 
o.o3 
6.34 
2.55 

2.65 
2.55 
o.o5 
3.o4 
i  .60 

6.58 
i.65 
1.16 

2.23 

1.87 

4.3i 

3.20 

3.06 

3.68 

I  .20 

14.24 
ii  .00 

2.74 

8.60 
2.83 

1  8.  oo 
20.72 
o.55 
18.68 
8.90 

13.91 
8.46 
i  .24 
ii  .61 

6.02 

18.27 
8.48 
5.90 
9.40 
2.08 

64.42 
48.66 
io.43 
48.29 
ig.83 

4.4i 
3.78 
0.74 

O.O2 
O.Og 

3.«4 
5.56 
2.59 
o.oo 

0.  II 

4.4o 
5.70 

2.05 
O.OI 

o.oo 

4.94 
3.93 
i  .39 
0.07 

O.OI 

3.68 
2.82 

I  .  IO 

o.63 
0.42 

3.92 
3.  4  1 
6.34 
2.o5 

3.18 

2.96 
4.17 

1.88 
4-7i 
4.42 

i4-  10 

9-77 
3.5o 

8.81 
6.28 

i4.oo 
i5.o8 
5.38 
o.o3 

0.20 

12.30 

10.16 
8.83 
2.75 
3.6i 

12.40 

10.17 
2.83 

I  I  .25 

9.76 

52.  80 

45.18 

20.54 

22.84 
19-85 

6.06 
2.93 

5.oi 
3.57 
3.56 

3.86 
3.92 
4.37 
4.22 
2.97 

4.22 

3.67 

4.32 

4.67 
3.34 

2.67 
3.52 
3.  10 
3.53 
2.68 

3.29 
3.55 
3.32 
3.i8 

2.87 

3.08 

3.09 

3.48 
3.36 

2.68 

2.68 

2.87 

4.29 
4.o3 
3.i3 

12.  3o 
10.45 

12.  l4 
10.97 

9.38 

i4.M 

10.52 

13.70 
12.46 

9.87 

8.94 
10.  16 
9.90 
10.07 
8.23 

6.94 

10.07 
ii  .  i5 
1  0.06 
7-48 

42.32 
4l  .20 

46.89 

43.56 
34.96 

3.46 

2.l5 

3.3o 
2.g5 
3.91 

3.i7 
3.99 
4.19 
i.83 

3.20 

4.7o 
o.64 
4-i4 
0.92 

2.18 

3.3i 
o.85 
3.88 
i.33 
3.3i 

3.4o 
i.57 
3.6o 
i  .26 

2.O4 

3.59 
3.85 
3.72 
i.37 
2.06 

3.93 
3.68 
3.43 
0.65 
i.3o 

H.55 
5.34 
n.o3 

8.69 
8.5i 

ii.33 
6.78 
n.63 
5.70 
9.29 

10.  3o 
6.27 
ii  .20 
3.96 

7-4i 

9-63 
6.90 
10.93 
1.63 

4.86 

42.81 
25.29 
44-79 
19.98 
30.07 

3.i3 
4.48 
i  .06 
4.oo 
3.25 

2.57 

4.39 
o.  16 
3.oo 

3.oi 

5.47 
3.44 
1.78 
2.80 
2.60 

4.27 

3.34 

2.34 
3.20 

3.o5 

3.73 
3.69 
7.3i 
i  ,4o 
3.39 

4.57 

3.24 
10.  27 
2.  10 

2.94 

4.3i 
2.91 

i4-43 

2.OO 
2.10 

io.85 

9-79 
19.12 
6.5o 
6.82 

11.17 

12.  3l 

3.oo 
9.70 

8.86 

12.57 
10.27 
19.92 
6.80 
9.38 

9.89 
8.3o 
29  59 
4.20 
5.38 

44-48 
40.67 
71.63 
27.20 
3o.44 

3.o4 
3.63 
4.82 
2.65 

2.85 

3.  72 
4.  ii 
3.27 
3.27 
o.oo 

2.81 
3.i8 
5.o4 
3.52 
i.i5 

4-46 
3.32 
3.70 
3.53 

1.87 

2.96 
i.35 
3.66 
3.87 
6.70 

2.91 

i.3i 
3.o4 
S.gS 

l3.20 

i  .5o 

o.67 
3.67 
4-4i 
6.  20 

7.16 
6.61 

i3.44 
8.62 
i6.43 

9.57 
10.92 

12.  l3 

9-44 
4.oo 

10.  33 
5.98 
io.4o 
H.35 

21.77 

4.29 
i  .92 

9-48 
11.81 

44.i5 

3i.35 
25.43 
45.45 

4l  .22 

86.35 

2.83 
1.97 
1.36 
6.67 
3.79 

3.75 
o.34 
7.72 
3.82 
4.i5 

3.39 
i.54 
2.57 
S.og 

7.81 

4.33 
2.67 
1.36 
5.79 
ii  .27 

3.35 
4-43 
4.4i 
7.88 

12.32 

3.08 
8.73 

3.86 
3.25 
8.5i 

2.21 
7.92 

3.36 
5.25 

8.65 

5.44 
11.19 
5.46 
12.69 
i8.32 

9-97 
3.85 
H.65 
14.58 
16.75 

10.76 
15.83 
9-64 
16.92 

32.10 

5.i8 
22.62 
9.71 

12.74 
23.77 

3i.35 
53.49 
36  46 
56-93 
89.94 

280 


TABLES  XXX.,  XXXI. — ANNUAL   FALL   OF  RAIN. 


TABLE  XXX. — PEACES  HAVING  A  BMALL  ANNUAL  FALL  OP  RAIN. 


Places. 

Latitude. 

Longitude. 

Height,  j  Ann.  nut. 

Authority. 

r 

0        ' 
—  12       0 
25    43 

25  54 
28  39 
3o     2 

O        ' 

77     2 
32  35 

—     l4     12 

6  46 
—  3i    i5 

Feet. 
53o 

i  ui'  in'.-. 

0 

o 

0 

o 
i.3i 

Arago's  Met.  Ess.,  p.  109. 
Wilk'n's  Egypt.,  v.  4,  p.  10, 
Gehler,  v.  7,  p.  i25i. 
Gehler,  v.  7,  p.  i25i. 
Arago  Melanges,  p.  463. 

"Thebes  E^ypt     '  

Near  Mourzouk,  Fezzan  . 
Eatta,  North  Africa  .... 
airo,  Egypt  

iKurrachee,  Hindostan  .  . 
jKotree,  Hindostan  

24  5o 

25    2O 

34  5i 
32  43 
46  21 

—  67     o 
—  68  14 
—     5  4o 
n4  36 

—  48     5 

35o 

120 

70 

i.5o 
1.74 
2.5o 

3.24 

4.o8 

Ph.  Trans.  i85o,  p.  36o. 
Ph.  Trans.  i85o,  p.  36i. 
An.  Met.  i854,  p.  297. 
Army  Reg.,  p.  676. 
Dove  Beitrage,  p.  i83. 

Biscara,  Algeria  

Fort  Yuma,  California  .  . 
Astrachan,  Russia  

Hyderabad,  Hindostan.  . 
Raimsk  Russia  

25    20 

46    4 
3g  53 

—  32    52 

44  27 

—  68  20 
—  61   4? 
-  44  33 
69     6 
—  5o     8 

260O 
26OO 

u5 

4.5o 
5.99 
6.i5 
6.5o 

6.72 

Johnston's  Ph.  Atlas. 
Dove  Beitrage,  p.  i83. 
Dove  Beitrage,  p.  i38. 
Zeitsch.  fur  Erd'e  1  858,  p.  9. 
Dove  Beitrage,  p.  i83. 

Aralich,  Russia  

Mendosa,  La  Plata  

Novo  Petrowsk,  Russia  . 

Fort  Conrad,  New  Mex.  . 
San  Louis  Rey,  Cal  
Barnaoul,  Siberia  

33  34 
33  i3 
53  20 
36  21 

10  27 

107     9 
117  3o 

—  83  27 
io5  42 
64  1  5 

4576 
20 
4oo 
8000 

0.76 
6.95 
7-4? 

7.48 

1     'Jo 

7  •  JJ 

Army  Reg.,  p.  673. 
Army  Reg. 
Kupffer's  Annales. 
Army  Reg. 
Gehler,  v.  7,  p.  i3i  i. 

Taos,  New  Mexico  

Cumana,  Venezuela  .... 

Sevastopol,  Russia  

44  36 
34  10 
44  57 

40    22 

32   1  3 
35     6 

33  32 
1  06  54 
—  34     6 
—  4g  47 
106  4s 
1  06  38 

456o 
780 
-53 

°'937 
5o32 

7.67 
7.86 
8.75 
9-o5 
9.23 
9.42 

Heis  Wochen't  1  866,  p.  325. 
Army  Reg.,  p.  673. 
Heis  Wochen't  1  858,  p.  1  76. 
Kupffer's  Annales. 
Army  Reg.,  p.  673. 
Army  Reg.,  p.  673. 

Socorro,  New  Mexico.  .  . 
Sympheropol,  Russia.  .  . 
Bacou,  Russia  

Fort  Filhnore,  N-^w  Mex. 
Albuquerque,  New  Mex. 

TABLE  XXXI. — PEACES  HAVING  A  GREAT  ANNUAL  FALL  OF  RAIN. 


Places. 

Latitude. 

Longitude-. 

Heigut. 

Amount. 

Authority. 

Cherapoonjee,  Hindost'n 
Matouba,  Guadeloupe  .  . 
Maranhao,  Brazil  

O       / 

25  14 
16  10 

—    2    3l 

8  39 
17  54 

O        r 

—  91  4o 
6  1   5o 

44  18 
—  77     o 
—  73  38 

Feet. 
4l25 

4ooo? 

45oo 
43oo 

Inches. 
592 
292 
280 
267 

264 

Herschel's  Met.,  p.  no. 
Com.  Rend.,  v.  7,  p.  743. 
Gehler,  v.  7,  p.  i3i4. 
Ph.  Trans.  i85o,  p.  358. 
Ph.  Trans.  i85o,  p.  354. 

Uttray  Mullay,  Hindost'n 
Mahabalishwar,  Hindo'n 

Sylket,  Hindostan  

24  53 
54 

20    4? 

8 

8    20 

—  91   47 
3 
—  93  25 

—  77 
i3     8 

1600 
6200 

209 
206 
200 

194 
189 

Br.  As.  1  852,  p.  257. 
Buchan's  Met,  p.  118. 
Buchan's  Met,  p.  1  1  7. 
Ph.  Trans.  i85o,  p.  362. 
Gehler,  v.  7,  p.  i3i4. 

Stye,  England  
Aracan,  Hindostan  

Augusta  Peak,  Hindost'n 
Sierra  Leone,  W.  Africa  . 

Sindola,  Hindostan  .... 
Vera  Cruz,  Mexico  .... 

19    12 

18  25 
16     3 
8 
—  20  5i 

-  73 
96     9 
—  94  3o 
-  97  38 

—  77 
—  55  3o 

46oo 

220O 

i85 
i83 
178 
i75 
170 
162 

Ph.  Trans.  i85o,  p.  354. 
Mayer's  Mexico. 
Br.  As.  1  852,  p.  z5j. 
Johnston's  Ph.  Atlas. 
Ph.  Trans.  i85o,  p.  362. 
Dove  Beitrage,  p.  1  02. 

Sandoway,  Hindostan  .  . 
Maulmein,  Birmah  

Attaghery,  Hindostan  .  . 
St.  Benoit,  Isl.  of  Bourbon 

Marmato,  New  Granada  . 
Demerara,  Guiana  

4  4o 
6  45 

IO    22 

20       8 

18  3o 
—  6  37 

74  42 
58     2 
67     5 
—  92  62 
72  3o 
—  i  06  49 

4678 

fix  en  en  en  en  cy. 
-a  o  en  en  o>  w 

Br.  As.  i84o,  p.  116. 
Berghaus's  Atlas. 
Dove  Beitrage,  p.  90. 
Br.  As.  1  852,  p.  257.    [368. 
Malte  Brun's  Geog.,  v.  i,  p. 
Dove  Beitrage,  p.  :o2. 

Caraccas,  Colombia  .... 
Akyab,  Hindostan  

Leogane,  St.  Domingo  .  . 
Buicenzorg,  Java  

TABLES   XXXII.,  XXXIII. — RADIATING  POWER,  ETC.        281 


TABLE  XXXII. 

COMPARATIVE  RADIATING  POWER  OP  DIFFERENT  SUBSTANCES  AT  NIGHT. 


Hare-skin  

i3i6 

Copper  .  . 

83o 

Rabbit-skin   

I24o 

Charcoal  in  Powder  

776 

White  raw  Wool  on  Grass.  . 

1222 

Wood  

7?3 

Flax  on  Grass  

1186 

Blackened  Tin 

77O 

Raw  Silk  

IIO7 

Lead  

757 

Unwrought  white  Cotton  ) 
Wool  \ 

io85 

Black-lead  in  Powder  
Zinc  

697 
681 

Yellow  Cotton  

1006 

Iron  

642 

Long  Grass  

IOOO 

Paper  

6i4 

Black  Wadding  on  Grass  .  . 

oo3 

Sawdust  

610 

Lampblack  in  Powder.  .... 

961 

Slate  

673 

Flannel  

886 

Garden-mould.  . 

4?2 

Light  blue  Lamb's  Wool  .  .  . 

876 

Tin-foil  

4?o 

Grass  less  than  an  Inch  in  ) 

River  Sand  

454 

Height,  .                         .  .  ( 

870 

Stone  

3qo 

Glass  

864 

Brick  

372 

Chalk  in  Powder  

84o 

Gravel  

288 

TABLE    XXXIII. 

FALL  OF  THE   BAROMETER  IN  HURRICANES. 


Locality. 

Date. 

Fall  in 
Inches. 

Hours. 

Authority. 

Near  Calcutta  .... 
Bay  of  Bengal  .... 
South  Indian  Ocean 
St.  Thomas,  W.  I.  .  . 
Near  Calcutta  .... 

(833,  May  21  . 
i84o,  Apr.  28. 
i84o,  May  4. 
1837,  Aug.  2. 
1  832,  Oct.  7. 

2.69 
2.o5 
2.OO 
I  .69 
I  .60 

3 

M 

6 

12 

Reid's  Law  of  Storms,  p.  271. 
Journal  Bengal  Soc.,  v.  9,  p.  1014. 
Piddington's  Horn-Book,  p.  21  5. 
Poggendorff  's  Annal.,  v.  52,  p.  25. 
Reid's  Law  of  Storms,  p.  269. 

Near  Hong  Kong  .  . 
Bay  of  Bengal  .... 
Bay  of  Bengal  .... 
China  Sea  

1867,  Sept.  8. 
1862,  May  i4. 
1  854,  Apr.  22. 
i845,  Oct.  9. 

i.S7 

i.55 
i  .5o 
i  .5o 

i3 

8 

12 

i3 

U.  S.  Steamer  Monocacy. 
Jour.  Bengal  Soc.,  1  85  5,  p.  429. 
Jour.  Bengal  Soc.,  i858,  p.  179. 
Jour.  Bengal  Soc.,  v.  18,  p.  16 

Mauritius  

i8i8,Feb.  28. 

i  .  5o 

17 

Reid's  Law  of  Storms,  p.  i4i. 

Havana,  Cuba  .... 
Macao,  China  

1  846,  Oct.  ii. 
1  832,  Aug.  3. 

1.47 
1.46 

0 
9 

Piddington's  Horn-Book,  p.  193. 
American  Journal,  v.  35,  p.  217. 

Calcutta 

1842,  June  3. 

i  .42 

18 

Jour.  Bengal  Soc.,  1842,  p.  ioo4 

Bay  of  Bengal  .... 
Aberdeen,  Scotland 

i85i,  Oct.  22. 
1839,  Jan.  7. 

i  .4o 

i  .  4<> 

7 

12 

Jour.  Bengal  Soc.,  i854,  p.  5i3. 
Espy's  Phil,  of  Storms,  p.  52  1  . 

Cape  Hatteras  .... 
Boston,  Mass  

1  853,  Sept.  7. 
1866,  Dec.  27. 

i.35 
i  .26 

7 
17 

Amer.  Journal,  v.  1  8,  N.  S.,  p.  9.    | 
R.  T.  Paine's  Journal. 

China  Sea  

1  809,  Sept.  28. 

I  .20 

12 

Jour.  Bengal  Soc.,  v.  1  1,  p.  627. 

Macao,  China  

1  835,  Aug.  5. 

T.T5 

8} 

American  Journal,  v.  35,  p.  211. 

Chittagong,  India.  . 
Mauritius  

1  849,  May  l3- 
1824,  Feb.  23. 

I  .06 

i  .o5 

«i 

5 

Jour.  Bengal  Soc.,  i854,  p.  22. 
Reid's  Law  of  Storms,  p.  i4?. 

282 


TABLE  XXXIV. — AURORAS,  SOLAK  SPOTS,  ETC. 


TABLE  XXXIV. 

ATTROBAS,  8OLAK  SPOTS,  AND  VARIATION   OF  THE  MAGNETIC  NEEDLE. 


Year. 

Auroras. 

Year. 

Auroras. 

Solar  Spots. 

Year. 

Auroras. 

Solar  Spots. 

u 

a 
> 

8P 

s 

Year. 

Auroras. 

Solar  Spots. 

1 

I 

1 

• 

« 
i 

H 

•c 
§ 

i 
§ 

3 

•< 

i685 

I 

1740 

2 

1782 

29 

24 

33 

8 

1824 

0 

o 

7 

8 

1686 

4 

I74l 

21 

i783 

17 

22 

22 

9 

l825 

I 

2 

17 

IO 

1692 

2 

1742 

i4 

2 

i?84 

7 

4 

4 

7 

1826 

2 

O 

29 

10 

1698 

2 

1743 

9 

2 

i785 

i4 

9 

18 

8 

1827 

IO 

7 

4o 

ii 

l6g4 

2 

1744 

8 

O 

1786 

40 

55 

61 

i4 

1828 

I  I 

6 

52 

12 

i6g5 

4 

I?45 

3 

O 

1787 

10 

47 

93 

i5 

1829 

18 

2 

53 

i4 

1696 

4 

1746 

i 

7 

1788 

10 

38 

91 

i3 

i83o 

32 

6 

59 

12 

1697 

i 

i74? 

7 

IO 

1789 

i5 

5i 

85 

i3 

i83i 

23 

2 

39 

12 

1698 

9 

i  ?48 

3 

6 

1790 

4 

i3 

75 

i5 

i832 

5 

2 

22 

1699 

4o 

1749 

3 

IO 

64 

1791 

4 

12 

46 

12 

i833 

12 

3 

7 

1702 

I 

1760 

12 

17 

68 

1792 

i 

6 

53 

9 

1  834 

2 

9 

II 

8 

1704 

I 

1761 

2 

5 

4i 

1793 

2 

8 

21 

8 

i835 

6 

6 

45 

IO 

1707 

12 

1762 

2 

33 

1794 

2 

2 

24 

8 

i836 

8 

5 

97 

12 

1708 

I 

i753 

I 

23 

1795 

2 

2 

16 

7 

1837 

25 

4i 

1  1  1 

12 

1709 

3 

i754 

0 

i4 

1796 

I 

O 

9 

8 

i838 

28 

39 

83 

13 

1710 

i 

i755 

I 

O 

6 

1797 

I 

O 

6 

8 

i839 

3o 

47 

68 

I  I 

1711 

i 

1766 

2 

O 

9 

1798 

0 

O 

3 

7 

i84o 

4o 

44 

52 

9 

1714 

i 

1767 

O 

6 

3o 

1799 

2 

O 

6 

7 

i84i 

35 

42 

3o 

7 

1716 

1  1 

i758 

2 

4 

38 

1800 

3 

O 

10 

7 

1842 

49 

1  1 

J9 

6 

1717 

12 

1769 

8 

5 

48 

1801 

4 

O 

3i 

8 

1843 

38 

10 

9 

7 

1718 

27 

1760 

7 

6 

49 

1802 

4 

2 

38 

8 

1  844 

22 

10 

i3 

6 

1719 

32 

1761 

12 

5 

?5 

i8o3 

6 

5 

5o 

9 

i845 

18 

22 

33 

7 

1720 

28 

1762 

18 

7 

5i 

i8o4 

6 

4 

70 

8 

1  846 

39 

3o 

47 

8 

1721 

19 

i763 

4 

6 

37 

i8o5 

4 

4 

5o 

9 

i847 

38 

22 

79 

9 

1722 

46 

i764 

9 

12 

34 

1806 

3 

4 

3o 

1  848 

38 

53 

IOO 

ii 

1728 

3o 

i765 

8 

7 

23 

1807 

o 

2 

IO 

1849 

42 

20 

96 

10 

1724 

26 

1766 

o 

o 

i? 

1808 

i 

0 

2 

i85o 

25 

3o 

64 

IO 

1726 

3o 

1767 

5 

4 

34 

1809 

0 

2 

I 

i85i 

'7 

21 

62 

8 

1726 

46 

1768 

2 

7 

52 

1810 

o 

0 

O 

i852 

45 

42 

52 

8 

1727 

67 

1769 

IO 

18 

86 

1811 

o 

O 

I 

i853 

26 

22 

38 

7 

1728 

86 

1770 

i3 

i4 

79 

1812 

o 

O 

5 

i854 

36 

i5 

'9 

7 

1729 

65 

1771 

29 

i5 

73 

i8i3 

o 

o 

i4 

7 

i855 

20 

7 

6 

1780 

116 

1772 

21 

7 

49 

1814 

4 

3 

20 

8 

i856 

2O 

4 

6 

1781 

57 

i773 

3i 

'7 

4o 

i8i5 

o 

i 

35 

8 

1857 

i5 

22 

7 

I732 

IOO 

1774 

48 

20 

48 

1816 

I 

o 

45 

i858 

34 

5l 

7 

1733 

27 

1776 

21 

5 

27 

1817 

I 

o 

44 

9 

i859 

46 

96 

10 

I?34 

38 

1776 

12 

4 

35 

1818 

2 

4 

34 

9 

1860 

33 

99 

IO 

1735 

5i 

1777 

26 

i5 

63 

1819 

3 

6 

22 

8 

1861 

35 

77 

9 

1736 

43 

1778 

3o 

18 

95 

1820 

2 

2 

9 

8 

1862 

33 

59 

8 

i737 

4o 

1779 

37 

4 

99 

1821 

2 

O 

4 

9 

i863 

36 

44 

8 

1738 

9 

1780 

2O 

25 

73 

1822 

O 

I 

3 

9 

1  864 

47 

46 

8 

1739   27 

1781 

29 

25 

68 

1823 

O 

0 

i 

8 

i865 

98 

3o 

7 

TABLE   XXXV. — CATALOGUE   OF   LARGEST  IROX   METEORS.    283 


TABLE  XXXV. 

CATALOGUE   OF   THE   LARGEST   IRON   METEORS. 


Locality. 

Year 
found. 

Pounds' 
Weight. 

Spec. 
Grav. 

Remarks. 

Durango,  Mexico  

l8ll 

35,ooo 

7   88 

Specimens  at  Berlin,  Vienna,  etc. 

Oturnpa,  Buenos  Ayres.  . 

Rogue  River,  Oregon.  .  . 
Bemdego  River,  Brazil  .  . 
Bonanza,  Mexico  

I?84 
1869 
I784 

i865 

33}ooo 

22,OOO 

i7,3oo 

sev'l  tons 

7.60 

7.73 
7.82 

\  Specimen  of  1400  Ibs.  belongs  to 
\      British  Museum. 
Specimens  at  Vienna,  Boston,  etc. 
Specimens  at  Munich,  London,  etc. 
Spec,  belongs  to  Prof.  C.  U.  Shepard. 

Near  Melbourne,  Aus.  .  . 
Sierra  Blanca,  Mexico  .  . 
Either0"  Prussia  

1861 
i?84 
1802 

8.287 
4,OOO 

3,4oo 

7.5i 
6.5o 
6.33 

Belongs  to  British  Museum. 
Specimen  at  Berlin. 
Specimens  at  Vienna,  Berlin,  etc. 

Near  Melbourne,  Aus.  .  . 
Zacatecas,  Mexico  

1861 
1792 

2,800 
2,000 

7.5, 

7  .5o 

Belongs  to  Colonial  Government. 
Specimens  in  Br.  Museum,  Berlin,  etc. 

Cocke  Co.,  Tennessee.  .  . 
Santa  Rosas,  New  Gran. 
Jenisey  River,  Siberia  .  . 
Red  River,  Texas  

i84o 
1810 
1772 
1808 

2.000 
1,700 

1,  680 

1,635 

7.26 
7.  3o 
6.48 

7.  7O 

Belongs  to  British  Museum. 
Specimens  at  Vienna,  Paris,  etc. 
Belongs  to  Imp.  Mus.,  St.  Petersburg. 
Belongs  to  Yale  College. 

Tucson,  Arizona,  U.  S.  .  . 

i735 

i.4oo 

Belongs  to  Smithsonian  Institute. 

La  Caille,  France  

iSflS 

1,100 

7.64 

Belongs  to  Jardin  des  Plantes,  Paris. 

Tucson,  Arizona,  U.  S.  .  . 
Tula  Russia          

i863 
T8/[6 

632 

542 

7.29 

5.  Q7 

Belongs  to  the  City  of  San  Francisco. 
Specimens  at  Vienna,  London,  etc. 

Bear  Creek,  Colorado  .  . 
Madoc,  Upper  Canada  .  . 

1866 
i854 

436 

368 

7.69 

Belongs  to  Prof.  C.  U.  Shepard. 
Geological  Cabinet  at  Montreal. 

Orange  River,  S.  Africa  . 
Cape  of  Good  Hope  .... 
Atacama  Bolivia  

i856 
i793 

l827 

326 
3oo 
3oo 

7-4o 
7.44 

Belongs  to  Prof.  C.  U.  Shepard. 
Belongs  to  Haarlem  Cabinet,  Holland. 
Belongs  mostly  to  British  Museum. 

Pittsburg,  Pennsylvania  . 
Carthage,  Tennessee.  .  .  . 

i85o 
1  846 

292 

280 

7.38 

Specs,  belong  to  Prof.  Silliman,  et  al. 
A  large  spec,  belongs  to  Brit.  Museum. 

Coahuila,  Mexico  

1  855 

262 

7.81 

Belongs  to  Smithsonian  Institute. 

Seelasfen  Silesia  

i847 

218 

7.  7O 

Belongs  partly  to  British  Museum. 

Toluca,  Mexico  

178/1 

218 

7.38 

150  Ibs.  belong  to  Prof.  C.  U.  Shepard. 

Brahin,  Russia  

1810 

200 

6.  20 

Belongs  to  University  at  Kiew. 

Lenarto,  Hungary  

1814 

ig4 

7.75 

Belongs  to  Museum  of  Pesth. 

Elbogen  Bohemia  

rHrr 

101 

7.74 

Chiefly  in  the  Cabinet  at  Vienna. 

Lion  River,  South  Africa 
Walker  Co.,  Alabama  .  . 
Nelson  Co.,  Kentucky  .  . 
Burlington,  New  York  .  . 

i853 
i832 
i856 
1819 

172 
i65 
161 
i5o 

7.60 
7.26 

7.60 

Belongs  to  Prof.  C.  U.  Shepard. 
Half  belongs  to  British  Museum. 
Specimens  in  Berlin,  London,  etc. 
Spec,  belongs  to  Prof.  C.  U.  Shepard. 

Ruff's  Mountain,  South  ~i 
Carolina  ) 

i85o 

116 

7.10 

Mostly  belongs  to  Prof.  C.  U.  Shepard.  • 

Lagrange,  Oldham  Co.,  | 
Kentucky  ) 

1860 

112 

7.8c 

Mostly  belongs  to  Prof.  C.  U.  Shepardj 

Bohumilitz,  Bohemia.  .  . 
A°ram  Croatia  

1829 
1761 

io3 

87 

7.60 
7.82 

Belongs  to  Museum  of  Prague. 
Belongs  chiefly  to  Vienna  Cabinet. 

Braunau,  Silesia  

1847 

72 

7.71 

Specimens  in  Vienna,  Berlin,  etc. 

Putnam  Co.,  Georgia.  .  . 
Tazewell,  Claiborne  Co.,\ 
Tennessee     / 

i83g 
i853 

70 
55 

7.69 
7.88 

Belongs  partly  to  Prof.  C.  U.  Shepard. 
Specimens  in  London,  Berlin,  etc. 

Schwetz  Prussia  

i85o 

43 

7.77 

Chieflv  in  Berlin. 

Denton  Co.,  Texas  

1  856 

4o 

7.67 

In  Geological  Cabinet  at  Austin. 

Claiborne,  Clarke   Co.,  ^ 
Alabama  ) 

i834 

4o 

6.  5o 

0 
Spec,  belongs  to  C.  T.  Jackson,  Boston. 

284     TABLE  XXXVI. — AEROLITES   FALLEN  IN   UNITED  STATES* 


TABLE   XXXVI. 

AEKOLITES   FALLEN   IN   THE   UNITED   STATES. 


Locality. 

Date  of  Fall. 

Weight  in 
Pounds. 

Specific 
Gravity. 

Present  Owners. 

Weston,  Conn.  .  .  . 
Caswell  Co.,  N.  C.  . 
Nobleborough,  Me. 
Nanjemoy,  Md.  .  .  . 
Simmer  Co.,  Tenn. 
Richmond,  Va.  .  .  . 

1807,  Dec.  1  4. 
1810,  Jan.  3o. 
1823,  Aug.  7. 
1825,  Feb.  10. 
1827,  May  9. 
1828.  June  4. 

3oo 
3 
5 
16 
ii 
4 

3.58 
3.5  ? 
3.09 
3.66 
3.55 
3.34 

Yale  College,  et  al. 
Unknown. 
C.  U.  Shepard,  et  al.         [al. 
Yale  Coll.  ;  C.  U.  Shepard,  et 
C.  U.  Shepard  ;  Leyden  Cab., 
C.  U.  Shepard,  et  al.     [et  al. 

Forsyth,  Monroe  / 
Co    Ga     .  .  .  .  f 

1829,  May  8. 

36 

3.46 

Yale  Coll.  ;  C.  U.  Shepard,  et 

Deal,  N.  J  

i82Q.Auer.  1  5. 

JL 

3.25 

C.  U.  Shepard,  et  al.         tal> 

Dickson  Co.,  Tenn. 
Little  Piney,  Mo.  . 
Bishopville,  S.  C.  . 
Linn  Co.,  Iowa  .  .  . 

i835,July3i. 
i839,Feb.i3. 
1  843,  Mar.  2  5 
i847,Feb.25. 

-j  «  en 
en  co  o  O 

a 

7.76 
3.5 
3.o4 

3.58 

Mobile  Cabinet,  et  al. 
C.  U.  Shepard,  et  al. 
C.  U.  Shepard,  et  al.         [al. 
Yale  Coll.  ;  C.  U.  Shepard,  et 

Castine  Me  

1  848,  May  20. 

X 

3.45 

Bowdoin  College,  et  al. 

Cabarrus  Co.,  N.  C. 
Petersburg,  Tenn.  . 
Harrison  Co.,  Ind.  . 
Bethlehem,  N.  Y.  . 
New  Concord,  O.  . 

1849,  Oct.  3i. 
i855,Aug.  5. 
i859,Mar.28. 
i85g,Aug.  ii. 
t86o,  May  i. 

18* 

4 

2 

it 

7OO 

3.63 

3.20 

3.46 
3.56 
3.54 

Yale  Coll.  ;  C.  U.  Shepard,  et 
C.  U.  Shepard,  et  al.         [al. 
C.  U.  Shepard,  et  al. 
Albany  Cabinet,  et  al.      [al. 
Marietta  Coll.  ;  Yale  Coll.,  et) 

Vernon  Co.,  Wis.  .  . 
Danville,  Ala  
Frankfort,  Ala.  .  .  . 
Stewart  Co.,  Ga.  .  . 
Searsmout,  Me.  .  .  . 
Waconda,  Kan.  .  .  . 

1  865,  Mar.  25. 
1  868,  Nov.  27. 
1  868,  Dec.  5. 
1  869,  Oct.  6. 
1  87  1,  May  25. 
1872, 

3.3 
4.5 
i.5 

1 

12 

88 

3.66 
3.4o 
3.3i 
3.65 
3.63 
3.70 

J.Lawrence  Smith. 
J.  Lawrence  Smith,  et  al. 
Yale  College,  et  al. 
Mercer  University,  et  al. 
Amherst  College,  et  al. 
Amherst  College,  et  al. 

Nash  Co.,  N.  C  
Iowa  Co.,  Iowa.  .  . 
Rochester,  Ind.  .  .  . 
Warrentou,  Mo.  .  . 
Cynthiana,  Ky.  .  .  . 
Emmet  Co.,  Iowa.  . 

1874,  May  1  4. 
i875,Feb.  12. 
1876,  Dec.  21. 
1877,  Jan.  3. 
1877,  Jan.  23. 
1  879,  May  10. 

4 
5oo 
i 
i5 
i3 
800 

3.6o 
3.57 
3.55 

3.47 
3.4i 
4.5o 

J.  Lawrence  Smith,  et  al. 
N.  R.  Leonard,  Yale  C.,  et  al. 
J.  Lawrence  Smith,  et  al. 
J.  L.  Smith,  Yale  Coll.,  et  al. 
J.  Lawrence  Smith. 
Br.  Mus.  Yale  Coll.,  et  aL 

EXPLANATION  OF  THE  TABLES. 


Table  L,  page  251,  contains  a  comparison  of  French  millimetres 
with  English  inches,  and  will  be  found  convenient  for  reducing 
French  measures  into  English.  It  is  deduced  from  the  assump- 
tion that  the  French  metre  at  the  freezing  point  is  equal  to 
39.37079  English  inches  at  the  temperature  of  62°  Fahrenheit, 
the  standard  temperature  of  the  French  scale  being  32°  Fahren- 
heit, and  that  of  the  English  scale  being  62°  Fahrenheit.  This 
is  the  result  given  by  Captain  Kater  in  the  Philosophical  Trans- 
actions for  1818,  page  109.  The  table  of  proportional  parts  in 
the  last  column  gives  the  value  of  tenths  of  a  millimetre  in  En- 
glish inches,  and  will  serve  for  hundredths  by  removing  the  deci- 
mal point  one  place  to  the  left. 

Table  II.,  page  252,  enables  us  to  convert  French  metres  into 
English  feet,  and  is  derived  from  the  same  data  as  the  preceding 
table ;  that  is,  the  French  metre  is  equal  to  3.2808992  English 
feet.  The  table  of  proportional  parts  in  the  last  column  may  be 
used  in  the  same  manner  as  described  in  Table  I. 

Table  III,  page  253,  enables  us  to  convert  French  kilometres 
into  English  miles,  and  is  derived  from  the  same  data  as  Table  I. ; 
that  is,  the  French  kilometre  is  equal  to  0.6213824  English  mile. 
The  table  of  proportional  parts  in  the  last  column  may  be  used 
in  the  same  manner  as  described  in  Table  I. 

Table  IV.,  page  254,  enables  us  to  convert  French  feet  into  En- 
glish feet.  The  old  legal  standard  of  France  was  the  Toise  de 
Perou,  so  called  from  its  being  used  by  the  French  academicians 
in  their  measurement  of  an  arc  of  the  meridian  in  Peru.  It  is 
formed  of  iron,  and  was  made  in  1735.  According  to  Base  du 
Syst&me  Metrique,  t.  iii.,  p.  237,  the  metre  is  equal  to  0.513074  toise, 


286  METEOROLOGY. 

or  3.078444  French  feet,  which  is  equal  to  3.2808992  English  feet 
Hence  one  French  foot  is  equal  to  1.065765  English  feet. 

The  arrangement  of  Table  IY.  is  similar  to  that  of  the  preced- 
ing tables.  The  same  table  will  serve  equally  well  for  converting 
French  inches  into  English  inches. 

Table  V.,  page  255,  enables  us  to  convert  degrees  of  the  cen- 
tesimal thermometer  into  degrees  of  Fahrenheit.  It  is  founded 

9 
on  the  equation  x°  centesimal—  (32°  +  -=x°)  Fahrenheit. 

o 

Table  VI.,  page  256,  enables  us  to  convert  degrees  of  Reau- 
mur's thermometer  into  degrees  of  Fahrenheit.  It  is  founded  on 

9 
the  equation  x°  Reaumur  =  (32°  +  -x°)  Fahrenheit. 


Table  VII.,  page  257,  gives  the  height  of  a  column  of  air  corr 
responding  to  a  tenth  of  an  inch  in  the  barometer  for  different 
temperatures  from  40°  to  90°,  and  may  be  used  for  reducing  ba- 
rometrical observations  to  the  level  of  the  sea,  or  to  any  other 
level. 

Example.  At  Cambridge,  Massachusetts,  at  70  feet  above  the 
sea,  the  mean  height  of  the  barometer  is  29.940  inches,  and  the 
mean  temperature  48°  ;  what  would  be  the  height  at  the  level  of 
the  sea? 

From  Table  VII.  we  find  for  barometer  29.94,  and  temperature 
48°,  the  number  90.8. 

Then  the  required  correction  equals  -f—  •  =  0.075. 


And  29.  940  +  .075  =  30.015  inches,  the  height  of  the  barometer 
at  the  level  of  the  sea. 

This  table  is  derived  from  Guyot's  Meteorological  Tables,  pub- 
lished by  the  Smithsonian  Institution,  D.  92. 

Table  VIII.,  pages  258-9,  gives  the  correction  to  be  applied  to 
English  barometers  with  brass  scales  for  reducing  the  observa- 
tions to  32°  Fahrenheit,  and  is  the  same  as  adopted  by  the  Royal 
Society  of  London.  From  29°  up  the  correction  must  be  sub- 
tracted from  the  observed  height,  while  from  28°  down  it  must  be 
added. 


EXPLANATION   OF  THE  TABLES.  287 

Example  1.  Observed  height  of  barometer,  29.876 ;  attached 
thermometer,  73°  Fahrenheit. 

On  page  259,  in  the  column  headed  30  inches,  on  the  horizon- 
tal line  corresponding  with  73°  in  the  first  vertical  column,  we 
find  the  correction  —.119.  Hence  the  barometer,  reduced  to  32° 
Fahrenheit,  will  be  29.876— .119  =  29.757  inches. 

Example  2.  Observed  height  of  barometer,  29.854 ;  attached 
thermometer,  17°  Fahrenheit. 

On  page  258,  under  30  inches  and  opposite  to  17°,  we  find  the 
correction  +.031.  Hence  the  barometer,  reduced  to  32°  Fahren- 
heit, will  be  29.854+. 031  =  29.885  inches. 

If  we  wish  the  correction  for  a  fraction  of  a  degree,  we  must 
take  a  proportional  part  of  the  difference  between  the  corrections 
for  the  nearest  whole  degrees  in  the  table. 

Table  IX.,  pages  260-1,  enables  us  to  compute  the  difference  in 
the  heights  of  two  places  by  means  of  the  barometer.  The  con- 
struction of  the  table  is  fully  explained  in  my  Introduction  to 
Practical  Astronomy,  page  480. 

Method  of  Computation. 

Take  from  Part  I.,  page  260,  the  two  numbers  corresponding 
to  the  observed  barometric  heights  h  and  h'.  From  their  differ- 
ence subtract  the  correction  found  in  Part  II.,  with  the  difference 
T — T'  of  the  thermometers  attached  to  the  barometers.  We  thus 
obtain  an  approximate  altitude,  a. 

We  then  calculate  the  correction  -          — a  for  the  tempera- 

yuu 

ture  of  the  air  by  multiplying  the  nine  hundredth  part  of  a  by 
the  sum  of  the  temperatures  t  and  t'  diminished  by  64.  This 
correction  is  of  the  same  sign  as  l  +  t'  —  64.  We  thus  obtain  a 
second  approximate  altitude,  A. 

With  A  and  the  latitude  of  the  place,  we  seek  in  Part  III.  the 
correction  arising  from  the  variation  of  gravity  with  the  latitude. 
With  A  we  also  seek  in  Part  IY.  the  correction  arising  from  the 
diminution  of  gravity  on  a  vertical.  Also,  when  the  height  of  the 
lower  station  is  considerable,  another  small  correction  is  found  in 
Part  V.  The  last  two  corrections  are  always  additive. 

Example.  The  following  observations  were  made  at  Geneva  and 
on  Mount  Blanc,  3.3  feet  below  the  summit  of  the  mountain. 


288 


METEOROLOGY. 


Mount  Blanc,  A'=16.695  inches,  T'=24°.4  Fah.,  *'  =  18°.3  Fah. 
Geneva,          h  =28.727      "      T  =65  .5     "    t  =66  .7    " 

j  for  h  =28.727  inches,      26476.8 
Fart  1.  gives  j  for  A/=16>695      «  12297.3 

Difference,      14179.5 

Part  II.  gives  for  T-T'=41°.l  -96.2 

Approximate  altitude,  a =14083.3 


—  =+328.6 
900      ^ 

Second  approximate  altitude,  A =14411.9 
Part  III.  gives  for  lat.  46°,  -  1.4 

Part  IV.  gives  for  14412,  +46.0 

Part  V.  gives  for  bar.  28.7,  +  1.6 

Sum, 

Height  of  Geneva  above  the  sea, 
Barometer  below  summit  of  Mt.  Blanc, 


14458.0 
1335.4 
3.3 
Height  of  Mount  Blanc  above  the  sea,  15796.7  feet. 


Table  X.,  page  262,  furnishes  the  mean  height  of  the  barometei 
fit  nine  stations  upon  the  American  continent;  also  at  nine  sta- 
tions in  the  western  part  of  the  Eastern  continent  ;  and  at  nine 
stations  in  the  eastern  part  of  that  continent.  The  precise  locali- 
ty of  these  stations  is  shown  in  the  following  table  : 


Georgetown,  Br.  Guiana 
Havana,  Cuba  

Latitude. 
6°  50' 
23      9 
31    31 
38    37 
39    58 
42    21 
43    40 
73    14 
78    37 
5    24 
12    50 
30      2 
41      0 
48    50 

Longitude. 
58°  12' 
82    23 
91    24 
90    15 
75    10 
71      3 
79    22 
88    56 
70    53 
0    16 

Greenwich,  England.. 
St.  Petersburg,  Russia 
Archangel,  Russia  
Hammerfest,  Lapland 
Singapore,  Malacca... 
Madras,  Hindostan  ... 
Bombay,  Hindostan.. 
Canton,  China  

Latitude 
51°  28' 
59    56 
64    32 
70    39 
1-  17 
13      4 
18    56 
23      8 
25    18 
39    54 
41    41 
51    18 
62      1 

Longitude. 
0°  0' 
30    18 

Natchez  Miss  

40  33 

23  42 

Philadelphia,  Penn  
Boston.  Mass  

-103  60 
80   19 

Toronto,  Canada  

-  72   54 
113   16 

Port  Bowen,  Arc.  Reg.  . 
Van  Rensselaer  Harbor. 
Christiansborg,  Africa  .. 
Aden,  Arabia  

Benares,  Hindostan  .. 
Pekin,  China  

-  82  56 
-116   26 
-  45   17 
119   20 

45      6 

Tiflis,  Georgia  

Cairo,  Egypt  

-31    15 
-29      0 
-  2    20 

Nertschinsk,  Russia... 
Jakutsk,  Siberia  

Constantinople,  Turkey. 
Paris.  France... 

-129  44 

A  portion  of  the  numbers  in  this  table  was  derived  from  an 
article  by  Professor  Dove,  published  in  the  Monatsberichte  der 
Akademie  zu  Berlin,  1860,  pages  644-692 ;  the  remainder  was 
derived  from  a  variety  of  sources. 


EXPLANATION   OF  THE   TABLES.  289 

Table  XI.,  page  263,  furnishes  the  mean  height  of  the  barome- 
ter for  all  hours  of  the  day  at  nine  stations  from  the  equator  to 
latitude  78°.  Most  of  these  places  are  included  in  the  preceding- 
list.  A  portion  of  these  numbers  was  derived  from  Kamtz's 
Lehrbuch  der  Meteorologie,  vol.  ii.,  pages  254-259 ;  the  others 
were  derived  from  various  sources. 

Table  XII.,  page  263,  furnishes  the  depression  of  mercury  in 
glass  tubes  on  account  of  capillarity,  according  to  several  differ- 
ent authorities. 

Table  XIII.,  page  264,  gives  the  weight  of  a  cubic  foot  of  dry 
air  and  of  satura-ted  air  under  a  barometric  pressure  of  30  inches, 
at  temperatures  between  0°  and  90°  F.  The  weight  of  a  cubic 
foot  of  dry  air  is  assumed  to  be  563  grains  troy  at  a  temperature 
of  32°  F.,  and  the  coefficient  of  expansion  is  0.002083  of  its  bulk 
for  1°  F. 

The  weight  of  a  cubic  foot  of  saturated  air  is  found  by  adding 
to  the  weight  of  a  cubic  foot  of  dry  air  the  weight  of  a  cubic  foot 
of  vapor,  and  correcting  this  result  for  the  enlargement  of  volume 
resulting  from  the  mixture.  This  table  is  derived  from  the  Green- 
wich Meteorological  Observations  for  1842,  pages  46  and  51. 

Table  XIY.,  page  265,  shows  the  height  of  the  barometer  cor- 
responding to  temperatures  of  boiling  water  from  188°  to  213*  F. 
The  temperature  at  which  water  boils  in  the  open  air  depends 
upon  the  weight  of  the  atmospheric  column  above  it,  and  under 
a  diminished  barometric  pressure  the  water  will  boil  at  a  lower 
temperature.  Since  the  weight  of  the  atmosphere  decreases  with 
the  elevation,  it  is  evident  that,  in  ascending  a  mountain,  the  high- 
er the  station,  the  lower  the  temperature  at  which  water  boils. 
Hence,  if  we  know  the  height  of  the  barometer  corresponding  to 
the  temperature  of  boiling  water,  we  can  measure  the  altitude  of 
a  mountain  by  observing  the  temperature  at  which  water  boils. 
This  table  is  copied  from  my  Practical  Astronomy,  page  398. 

Table  XV.,  page  266,  gives  the  corrections  to  be  applied  to  the 
means  of  the  hours  of  observation  to  obtain  the  true  mean  tem- 
perature at  New  Haven.  These  numbers  are  the  differences, 
with  opposite  signs,  between  the  hourly  temDeratures  and  the 

T 


290  METEOROLOGY. 

true  mean  temperature  of  each  month  and  also  of  the  year. 
Thus,  at  New  Haven,  the  mean  temperature  of  January  is  26°.5; 
the  mean  temperature  at  midnight  in  January  is  24°.2 ;  the  dif- 
ference is  2°.3,  which  is  the  quantity  which  must  be  added  to 
midnight  observations  to  obtain  the  mean  temperature  of  that 
month,  and  so  for  the  other  hours  and  months  of  the  table. 

At  the  bottom  of  the  table  is  given  a  comparison  of  some  of 
the  different  modes  which  have  been  proposed  for  deducing  the 
mean  temperature  from  a  limited  number  of  observations.  Thus, 
if  we  have  observations  at  7  A.M.  and  1  P.M.  in  January,  the 
former  require  a  correction  of  -f  4°.4  and  the  latter  of  —  6°.l ;  the 
mean  of  the  two  will  require  a  correction  of  —  0°.8,  as  given  in 
line  26th  of  the  table. 

If  we  have  observations  at  6  A.M.,  2  and  6  P.M.  in  January, 
the  corrections  for  these  three  hours  will  be  +4°.3,  —  6°.3,  and 
— 1°.4.  The  mean  correction  is  —  l°.l,  which  is  the  number  given 
in  line  36th  of  the  table. 

If  we  have  observations  at  7  A.M.,  2  and  9  P.M  ,  and  if  we  add 
twice  the  nine  o'clock  observation  to  the  sum  of  the  other  two 
observations,  and  divide  the  result  by  4,  the  error  of  the  result 
for  the  separate  months  in  only  one  instance  exceeds  a  quarter  of 
a  degree.  This  table  is  copied  from  the  Transactions  of  the  Con- 
necticut Academy  of  Arts  and  Sciences,  vol.  i.,  p.  231. 

Table  XYL,  page  267,  is  constructed  for  Greenwich,  England, 
in  the  same  manner  as  the  preceding,  and  is  taken  from  the  Green- 
wich Meteorological  Observations. 

Table  XVII.,  pages  268-9,  gives  the  mean  temperature  of  45 
places  on  the  American  continent  for  each  month  of  the  year. 
Some  of  these  numbers  are  derived  from  Dove's  Tables  in  the 
Report  of  the  British  Association  for  1847,  page  376;  others  are 
derived  from  the  Army  Meteorological  Register,  1855,  and  some 
from  other  sources. 

Table  XVIIL,  page  270,  furnishes  a  list  of  places  whose  mean 
temperature  is  above  80°  F.  The  materials  are  derived  chiefly 
from  Dove's  Tables. 

Table  XIX.,  page  270,  furnishes  a  list  of  places  whose  mean 


EXPLANATION   OF   THE   TABLES.  291 

temperature  is  below  18°  F.     This  is  also  derived  chiefly,  but  not 
exclusively,  from  Dove's  Tables. 

Table  XX.,  page  271,  furnishes  a  list  of  places  where  the  mean 
temperature  of  the  hottest  month  differs  less  than  six  degrees 
from  that  of  the  coldest  month,  and  is  chiefly  derived  from  Dove's 
Tables. 

Table  XXL,  page  271,  furnishes  a  list  of  places  where  the  mean 
temperature  of  the  hottest  month  differs  more  than  sixty-six  de- 
grees from  that  of  the  coldest  month.  The  materials  are  derived 
partly  from  Dove's  Tables,  partly  from  Kupffer's  Annales,  and 
partly  from  other  sources. 

Table  XXIL,  page  272,  furnishes  a  list  of  places  where  the  an 
nual  range  of  temperature  is  less  than  40°.     The  materials  were 
derived  partly  from  Arago's  Works,  vol.  viii.,  pages  184-646,  but 
many  of  the  numbers  were  obtained  by  an  extensive  comparison 
of  Meteorological  Journals. 

Table  XXIII.,  page  272,  furnishes  a  list  of  places  where  the 
annual  range  of  temperature  is  greater  than  130°.  The  materials 
were  derived  partly  from  Arago,  vol.  viii.,  but  many  of  the  num- 
bers were  obtained  by  an  extensive  comparison  of  Meteorologies* 
Journals,  particularly  Kupflfer's  Annales,  the  Army  Meteorolog- 
ical Kegister,  and  the  New  York  Meteorological  Observations. 

Table  XXIV.,  page  273,  shows  the  height  of  the  line  of  per- 
petual snow  above  the  level  of  the  sea  for  a  variety  of  latitude* 
The  works  chiefly  depended  upon  in  preparing  this  table  are  the 
Encyclopaedia  Metropolitans,  Art.  Meteorology,  page  84 ;  Miiller's 
Lehrbuch  der  Kosmischen  Physik,  page  353 ;  and  Kaemtz's  Me- 
teorology. 

Table  XXV.,  page  273,  contains  the  factors  by  which  the  dif- 
ference of  readings  of  the  dry-bulb  and  wet-bulb  thermometers 
must  be  multiplied  in  order  to  produce  the  difference  between 
the  readings  of  the  dry -bulb  and  dew-point  thermometers.  These 
factors  are  derived  from  a  long  series  of  observations  made  at  the 
Greenwich  Observatory,  and  enable  us  to  convert  observations 


292  METEOROLOGY. 

made  with  the  wet-bulb  thermometer  into  observations  made 
with  Daniell's  hygrometer. 

Example.  The  temperature  of  the  air  being  44°.5,  and  that  of  the 
wet-bulb  being  38°.7,  it  is  required  to  determine  the  dew-point. 

The  difference  between  the  dry  and  wet  bulb  thermometer  is 
5°.8,  which,  multipled  by  2.17,  gives  12°. 6,  which  is  the  difference 
between  the  dry-bulb  and  dew-point  thermometers.  Hence  the 
dew-point  was  at  31°.9. 

Table  XXVI.,  pages  274-5,  shows  the  relative  humidity  of  the 
air  at  temperatures  from  6°  to  95°,  and  for  a  difference  of  tem- 
perature of  air  and  of  the  dew-point  from  0°  to  24°.  The  rela- 
tive humidity  is  the  ratio  of  the  quantity  of  vapor  actually  con- 
tained in  the  air  to  the  quantity  it  could  contain  if  fully  satu- 
rated, Art.  105.  This  humidity  is  deduced  from  Table  XXVII. 

Example.  Suppose  the  temperature  of  the  air  is  90°  F.,  and 
that  of  the  dew-point  is  80°  F.,  the  difference  being  10°  F.  Ac- 
cording to  Table  XXVII.,  the  elastic  force  of  vapor  at  these  two 
temperatures  is  1.410  and  1.023 ;  their  ratio  is  .73,  which  is  the 
relative  humidity,  and  is  the  number  given  in  the  table  for  a  tem- 
perature of  90°,  and  a  dew-point  10°  below  the  temperature  of  the 
air.  Making  the  point  of  saturation  100,  all  the  numbers  in  the 
table  are  to  be  regarded  as  integers.  This  table  is  abridged  from 
one  given  in  the  Smithsonian  Meteorological  Tables,  B.  75. 

Table  XXVII.,  page  276,  gives  the  elastic  force  of  aqueous  va- 
por for  temperatures  from  —30°  to  101°  F.,  according  to  the  ex- 
periments of  Regnault.  The  table  is  abridged  from  the  Smith- 
sonian Tables.,  B.  43. 

Table  XXVIIL,  page  277,  is  designed  to  furnish  a  comparison 
between  the  pressure  and  velocity  of  the  wind.  It  is  derived 
from  the  Meteorological  Papers  of  the  British  Board  of  Trade, 
third  number,  page  99,  and  was  computed  by  Colonel  James,  as- 
suming that  the  square  of  the  velocity  in  miles  per  hour,  multi- 
plied by  0.005,  gives  the  pressure  in  pounds  per  square  foot. 
These  numbers  differ  slightly  from  those  given  on  page  70,  but 
neither  table  can  be  regarded  as  perfectly  reliable.  More  numer- 
ous experiments  are  needed  for  determining  the  pressure  of  the 
wind  at  different  velocities. 


EXPLANATION   OF   THE   TABLES.  293 

Table  XXIX.,  pages  278-9,  gives  the  average  amount  of  rain 
for  each  month  of  the  year  at  45  stations  on  the  American  conti- 
nent, extending  from  near  the  equator  to  the  highest  northern 
latitude  for  which  such  observations  could  be  found.  A  consid- 
erable part  of  these  results  is  taken  from  the  Army  Meteorologi- 
cal Eegister,  published  in  1855;  the  remainder  are  chiefly  de- 
rived from  Dove's  Klimatologische  Beitrage,  and  the  Meteorolog- 
ical Observations  of  the  Smithsonian  Institution,  while  a  few  have 
been  derived  from  other  sources. 

Table  XXX.,  page  280,  gives  a  list  of  stations  at  which  the  an- 
nual fall  of  rain  is  less  than  ten  inches.  The  table  furnishes  the 
authorities  for  the  results  here  given. 

Table  XXXI.,  page  280,  gives  a  list  of  stations  at  which  the 
annual  fall  of  rain  exceeds  twelve  feet.  These  stations  have  gen- 
erally considerable  elevation  above  the  sea,  but  in  many  of  the 
cases  the  heights,  not  being  accurately  known,  could  not  be  given 
in  the  table. 

Table  XXXIL,  page  281,  shows  the  comparative  radiating 
power  of  different  substances  at  night,  according  to  the  observa- 
tions of  Mr.  Glaisher,  made  at  Greenwich,  England,  and  published 
in  the  Philosophical  Transactions  for  1847,  page  119.  The  num- 
bers refer  to  long  grass  as  the  unit. 

Table  XXXIII.,  page  281,  shows  the  fall  of  the  barometer  dur- 
ing several  remarkable  hurricanes  in  the  West  Indies,  the  East  In- 
dies, and  elsewhere.  The  table  shows  the  fall  in  the  number  of 
hours  given  in  column  fourth,  but  this  is  not  generally  the  entire 
fall  of  the  barometer  during  the  day  of  the  hurricane,  for  the  high- 
est point  of  the  barometer  usually  occurs  some  hours  before  the 
rapid  fall  begins,  or  some  hours  after  the  most  rapid  rise  at  the 
close  of  the  storm. 

Table  XXXIV.,  page  282,  gives  a  catalogue  of  auroras  ob- 
served in  Europe  since  1685,  and  in  America  since  1742,  the  lat- 
ter being  chiefly  confined  to  Boston  and  New  Haven.  These  num 
bers  show  clearly  the  unequal  frequency  of  auroras  in  the  different 
years,  and  these  inequalities  indicate  a  period  of  ten  or  twelve 


294  METEOROLOGY. 

years,  with  a  more  decided  period  of  about  sixty  years.  The  ta- 
ble also  shows  the  relative  frequency  of  solar  spots  since  1749, 
and  the  mean  daily  range  of  the  magnetic  needle  as  observed  in 
Europe  since  1782.  It  will  be  noticed  that  the  last  two  phenom- 
ena show  most  decided  periodic  inequalities,  and  these  periods 
correspond  remarkably  with  the  periods  of  auroral  abundance. 
The  table  is  abridged  from  several  tables  published  in  the  Smith- 
sonian Report  for  1865,  pages  225-243. 

Table  XXXV.,  page  283,  gives  a  catalogue  of  the  principal  iron 
meteors  exceeding  40  pounds  in  weight.  It  is  not  claimed  that 
ibis  catalogue  is  complete,  for  in  the  report  of  many  meteors  the 
weight  is  not  definitely  stated.  The  number  of  iron  meteors 
whose  weight  is  less  than  40  pounds  is  nearly  equal  to  the  num- 
ber embraced  in  this  catalogue.  This  catalogue  has  been  com- 
piled from  a  great  variety  of  sources,  but  chiefly  from  Buchner's 
Meteoriten,  1863. 

Table  XXXVI.,  page  284,  gives  a  list  of  the  aerolites  fallen 
in  the  United  States.  Besides  these,  there  are  five  or  six  other 
cases  in  which  aerolites  have  been  claimed  to  have  fallen,  but  as 
those  cases  are  not  considered  to  be  sufficiently  well  attested  they 
have  been  omitted. 


EXPLANATION  OF  THE  PLATES, 


PLATE  I.  shows  the  prevalent  winds  at  eight  stations  of  the 
American  continent  from  near  the  equator  to  latitude  78°  N. 
Horizontal  and  vertical  lines  are  drawn  to  represent  the  four 
cardinal  points,  and  diagonal  lines  are  drawn  for  the  intermediate 
directions.  On  these  eight  lines  are  set  off  distances  correspond- 
ing to  the  relative  frequency  of  the  winds  from  these  eight  quar- 
ters. The  curve  line  passing  through  the  eight  points  thus  de- 
termined may  be  regarded  as  showing  the  prevalent  wind  at  that 
station.  It  is  thus  seen  that  at  Van  Rensselaer  Harbor  and  at 
Godthaab  the  prevalent  wind  is  from  the  N.E. ;  at  Norway 
House  it  is  from  the  north ;  at  St.  Johns,  New  York,  and  Savannah 
the  prevalent  wind  is  from  the  S.W. ;  at  Matanzas  it  is  from  the 
N.E. ;  and  at  Georgetown  it  is  nearly  from  the  east. 

Plate  II.  represents  the  six  varieties  of  cloud  described  on 
pages  101  and  102,  each  variety  being  indicated  by  a  symbol 
shown  at  the  bottom  of  the  page. 

Plate  III.  exhibits  two  outline  maps  of  the  United  States,  de- 
signed to  illustrate  the  phenomena  of  a  storm  described  on  pages 
142  and  143. 


WORKS  ON  METEOROLOGY. 


THE  student  who  desires  a  more  thorough  knowledge  of  me- 
teorology than  can  be  obtained  from  this  treatise,  is  referred  to 
the  following  works  and  memoirs : 

Annales  de  1'Observatoire  Physique  Central  de  Eussie.  One 
large  quarto  volume  of  observations  annually. 

Annuairc  Magnetique  et  Meteorologique  du  Corps  des  Inge- 
nieurs  des  Mines  de  Russie. 

Annuaire  de  la  Societe  Meteorologique  de  France.  One  large 
octavo  volume  annually  since  1849. 

Apjohn.  Theory  of  the  Moist-bulb  Hygrometer,  Edinb.  Philos. 
Transact,  xvii. 

Arago.  (Euvres  Completes,  12  volumes,  8vo.  Etat  thermome- 
trique  du  Globe  terrestre. — Influence  de  la  Lune. — La  pluie. — Le 
tonnere. — Puits  Artesiens,  etc. 

Baddeley.  On  Dust  Whirlwinds  and  Cyclones  in  India. 

Biot.  On  Mirage  and  unusual  Refraction,  Mem.  de  1'Academie. 
1809. 

Birt.  Reports  to  the  British  Assoc.  on  Atmospheric  "Waves^ 
1844-8. 

Blodget.  Climatology  of  the  United  States.     Philadelphia,  1857 

Boue.  Katalog  der  Nordlichter  bis  1856.    WienAcad.,  74  pages. 

Bravais  and  Martin.  Comparaisons  Barom.  faites  dans  le  Noid 
de  1'Europe,  Mem.  Acad.  Brux.,  xiv. 

Brewster.  On  the  Mean  Temperature  of  the  Globe,  Edinb.  Phil. 
Transact.,  ix.  Results  of  Thermometrical  Observations  at  Leith 
Fort,  do.,  x. 

Buchan.  Handy  Book  of  Meteorology.     London,  1867. 

Buchner.  Meteoriten.     Leipsig,  1863. 

Bulletin  de  1'Observatoire  de  Paris.  One  sheet  daily,  contain- 
ing Meteorological  Reports  from  every  part  of  Europe. 

Buist.  Catalogue  of  Indian  Hailstones  and  Meteors. 


WORKS   ON   METEOROLOGY.  297 

Buys  Ballot  Sur  la  marche  annuelle  du  thermometre  et  du  ba- 
rometre  en  divers  lieux  de  1'Europe,  1849-59,  Amsterd.  Acad., 
1861, 116  pages. 

Cordier.  On.  Temperature  of  Interior  of  the  Earth,  Mem.  Acad 
Sci.,  1827. 

Correspondence,  Met.  de  1'Obs.  Central  Physique  de  Russie.  A 
thin  quarto  volume  annually. 

Code.  Meteorologie,  Paris,  1774. 

Daguin.  Traite  de  Physique,  avec  les  applications  a  la  Meteo- 
rologie, 4  vols.,  Paris,  1861. 

Dallon.  On  Rain  and  Dew,  Manchester  Mem.,  v.  On  the 
Constitution  of  Mixed  Gases,  do.  Met.  Obs.  and  Essays,  London. 
On  Constitution  of  the  Atmosphere,  Phil.  Trans.,  1826.  On 
Height  of  Aurora  Borealis,  Phil.  Trans.,  1828. 

Dani.ell.  Meteorological  Essays,  2  vols.  8vo,  London.  On  the 
Constitution  of  the  Atmosphere. 

De  Luc.  On  Hygrometry,  Ph.  Trans.,  1791.  On  Evaporation, 
do.,  1792. 

Dove.  Tables  of  Mean  Temperature,  Rep.  of  Br.  Assoc.,  1847. 
Verbrvitung  der  Warrne  auf  der  Oberflache  der  Erde,  4to.  Kli- 
matologische  Beitrage,  1861.  Das  Gesetz  der  Stiirme,  2d  edit. 

Ermann.  L^eber  Boden  und  Quellen  Temperatur.  Ueber 
einig'-  Barom.  Beob.,  Poggendorff,  Ixxxviii. 

Espy.  Philosophy  of  Storms,  Boston,  1841.  Reports  on  Me- 
teorology of  U.  S. 

Fitzroy.  Weatherbook,  a  Manual  of  Practical  Meteorology. 
London,  1863. 

Forbes.  Report  to  Brit.  Assoc.  on  Meteorology,  1832.  Supple- 
mentary Report,  1840.  On  the  Climate  of  Edinburg  for  56  years, 
Trans.  R.  S.  Edinb.,  xxii.,  part  2. 

Frit-ych.  Periodische  Erscheinungen  in  Wolkenhimmel,  R.  Bo 
hem.  Acad.,  v.  Folge,  Bd.  iv. 

Gallon.  Meteorographica,  or  Maps  of  the  Weather,  4to.  Lon- 
don, 1863. 

Gehkr.  Worterbuch,  Arts.  Meteorologie,  Regen,  etc. 

Glaisher.  On  Nocturnal  Radiation,  Phil.  Trans.,  1847.  On  Cor- 
rection of  Monthly  Means  of  Met.  Obs.,  Phil.  Trans.,  1848. 

Greg.  On  Aerolites,  L.,  E.,  and  Dub.  Phil.  Mag.,  1854,  p.  329. 
Catalogue  of  Meteorites,  Rep.  Br.  Assoc.,  1860,  p.  48. 

Guyot.  Meteorological  Tables,  8vo,  1859. 


298  WORKS  ON  METEOROLOGY. 

Harvey.  Art.  Meteorology,  Encyc.  Metropolitana. 

Heis.  Ueber  Sternschnuppen.     Koln,  1849. 

Herschel,  J.  F.  W.  Admiralty  Manual  of  Scientific  Inquiry.  Lon« 
don,  1851.  Meteorology,  1862. 

Hopkins.  On  Winds  and  Storms.     London,  1860. 

Hough.  New  York  Meteorological  Observations,  1826-50.  Al- 
bany, 1855. 

Howard.  Climate  of  London,  3  vols.,  8vo.  On  a  Met.  Cycle  of 
18  years,  Phil.  Trans.,  1841.  Barornetrograpliia. 

Hudson.  Hourly  Obs.  of  the  Barometer,  Phil.  Trans.,  1832. 

Humboldt.  On  Isotherms,  Mern.  d'Arceueil,  iii.  On  Inferior 
Limit  of  Perpetual  Snow,  Ann.  de  Chim.,  xiv.  Kosmos. 

Jelinek.  Tagliche  Gang  der  Meteorolog.  Elemente.    Wien. 

Johnson.  Met.  Obs.  at  Radcliffe  Observatory,  Oxford. 

Johnston,  Keith.  Physical  Atlas  of  Natural  Phenomena. 

Kamtz.  Lehrbuch  der  Meteorologie.  Leipsig,  3  vols.  On  Iso- 
barometric  Lines.  Meteorology  translated  C.  V.  Walker. 

Kirkwood.  Meteoric  Astronomy.     Philadelphia,  1867. 

Koller.  Gang  der  Warme  in  Oesterreich  (Kremsmunster,  1841). 

Kupffer.     On  Springs  and  Earth  Temp.,  PoggendorfF,  xx. 

Lamont.  Beobachtungen  aufd.  Hohenpeissenberg,  1792-1850. 
Miinchen,1851.  Annalen  fur  Meteorologie. 

Lawson.  Army  Meteor.  Register  for  12  years,  1843-54.  Wash- 
ington, 1855. 

Loomis.  On  two  Storms  which  occurred  in  February,  1842, 
Trans.  Am.  Phil.  Soc.,  vol.  ix.  On  the  Storm  of  December,  1836, 
Smith.  Contrib.,  1860.  On  the  Aurora  Borealis,  Smith.  Report, 
1865,  p.  208.  Mean  Temperature  of  New  Haven,  Conn.,  Trans. 
Conn.  Acad.,  vol.  i. 

Mahlmann.  Temperature  auf  der  Oberflache  der  Erde  (Dove's 
Repertorium,  Bd.  iv.). 

Mairan.  Traite  de  1'Aurore  Boreale,  2d  ed.,  Paris. 

Maury.  Storm  and  Rain  Charts  of  the  North  and  South  At- 
lantic. 

Meech.  Relative  Intensity  of  the  Heat  and  Light  of  the  Sun 
upon  different  Latitudes,  Smith.  Contr.,  1855. 

Meteorological  Society  (of  London).  Transactions  and  Council 
Reports. 

Muhry.  Klimatologische  Untersuchungen,  1858. 

Muller.  Lehrbuch  der  Kos mi sch en  Physik. 


WORKS   OX   METEOROLOGY.  299 

Newton.  On  Shooting  Stars,  Mem.  Nat.  Acad.  Sciences,  vol.  i. 
Original  Accounts  of  November  Star  Showers,  Am.  Jour.  Science, 
N.  S.,  vol.  xxxvii.,  p.  377.  Contributions  to  Astro-Meteorology, 
Journ.  Sc.,  vol.  xliii.,  p.  285,  etc. 

Olmsted.  Secular  Period  of  the  Aurora  Borealis,  Smith.  Contr., 
1855. 

Partsch.  Die  Meteoriten.     Wien,  1843. 

Peltier.  Sur  les  Trombes.     Paris. 

Phipson.  Meteors,  Aerolites,  and  Falling  Stars.     London,  1867. 

Piddington.  Nineteen  Memoirs  on  Cyclones  in  the  Indian  r.nd 
China  Seas. — Sailor's  Horn-Book. 

Plantamour.  Des  Anomalies  de  la  Temperature  a  Geneve,  1867. 
Resume  des  Obs.  Therm,  et  Bar.  a  Geneve. 

Pouillet.  Mem.  sur  la  Chaleur  Solaire,  Comptes  Rendus,  1838. 

Quetelet.  Sur  le  Climat  de  la  Belgique.  Catalogue  des  Appari- 
tions des  Etoiles  Filantes,  Acad.  Brux.,  1839.  Variations  Pe- 
riodiques  de  la  Tern.,  Acad.  Brux.,  xxviii.  Meteorologie  de  Belg 

Reid.  Law  of  Storms.     On  Storms  and  Variable  Winds. 

Redfield.  On  the  Courses  of  Whirlwinds.  Am.  Journ.  Sc., 
xxxv.,  etc. 

Robinson.  Improved  Anemometer,  Royal  Irish  Academy,  xxii. 

Saline.  Report  on  Meteorology  of  Toronto,  Br.  Assoc.,  1844. 
Lunar  Tide  at  St.  Helena,  Phil.  Trans.,  1847.  Meteorology  of 
Bombay,  Phil.  Trans.,  1853.  Variations  of  Temperature  at  To- 
ronto, Phil.  Trans.,  1853. 

Saussure.  Essais  de  1'Hygrometrie.     1783. 

Schlagintweit.  Results  of  a  Scientific  Mission  to  India,  1854-8, 
4  vols.  4to,  Leipsig. 

Schouiv.  Beitrage  zu  Vergleichenden  Klimatologie,  Bibl.  U.T 
xxxiv. 

SchuUer.  Atmospheric  Electricity.  Jahrbuch  der  Chem.  und 
Phys.,  1829. 

Secchi.  Results  of  Met.  Obs.  at  Rome,  Bibl.  U.,  1857. 

Sykes.  On  Atmospheric  Tides,  Phil.  Trans.,  1835  Observations 
in  India,  Phil.  Trans.,  1850. 

Thomson.  Introduction  to  Meteorology.    London  1849. 

Welsh.  Account  of  four  Balloon  Ascents,  Phil.  Trans.,  1856. 

Wells.  On  Dew.     London  1818. 

Whewell.  On  a  new  Anemometer,  Trans.  Cam.  Phil.  Soc.,  vi. 

Wollaston.  On  the  finite  Extent  of  the  Atmosphere,  Phil.  Trans 


300  WORKS  ON  METEOROLOGY. 

Coffin,  Winds  of  the  Northern  Hemisphere,  Smithsonian  Con- 
tributions, vol.  vi. 

Fen-el  Motions  of  Fluids  and  Solids  relative  to  the  Earth's 
Surface,  Math.  Monthly,  vols.  i.  and  ii. 

Herrick.  Eegister  of  the  Aurora  Borealis,  Transactions  of  the 
Connecticut  Academy,  vol.  i. 

Quetelet.  Meteorologie. 

Smithsonian.  Meteorological  Kesults,  1854-9. 


INDEX. 


Aerolite  a"  Braunau,  Bohemia 

"       at  Guernsey,  Ohio 

"        at  Orgueil,  France 

"        at  Weston,  Conn 

Aerolites,  composition  of 

conclusions 

described 

formed  in  our  atmosphere 

from  lunar  volcanoes 

"     terrestrial  volcanoes. 

in  the  United  States 

number  of. 

peculiarities  of 

periodicity  of. 

Air  exerting  one  tenth  inch  pressure. 

Anemometer,  Osier's 

"  Robinson's 

Whewell's 

Woltmann's 

Anemoscope  described 

"  self-registering 

Anomalous  months,  conclusions  from 
Arcs  intersecting  opposite  the  sun.... 

"    touching  halo  of  46° 

Atlantic  Ocean,  two  sides  of. 

Atmosphere,  actual  height  of. 

composition  of. 

heated,  how 

height  deduced  from  twi- 
light  

limit  of,  estimated 

regulates  temperature. . 

upper  half  of. 

"     regions  of. 

Atmospheric  circulation,  system  of... 

August  meteors  described 

meteors,  elements  of. 

stream,  dimensions  of. 

Au  ora  caused  by  nebulous  matter... 

colors  of,  described 

"       explained 

conflicting  estimates , 

height  of ... 

noise  of. 

polaris 

varieties  of. 

Auroral  arches,  anomalous  forms  of. . 


Page  Pan 

242  Auroral  arches,  anomalous  position 

242  of 196 

243  Auroral  arches,  breadth  of. 179 

241  "          "       described 174 

244  "          "       form  of. 179 

249  "          "       movements  of. 180 

241  '  "       position  described ....  179 

247  '  "             "       explained...   196 

247  '          "       structure  of. 181 

247  '       beams,  cause  of. 196 

284  '          "       described 175 

243  '          "       explained 194 

244  '          "       motion  of. .,  182 

246  '       clouds 183 

259  '      corona  described 175 

68  '            "      explained 193 

67  '            "      position  of. 182 

66  '       exhibitions,  terrestrial 191 

66  '       flashes,  cause  of. 197 

65  "       light  is  electric  light 192 

66  "      vapor 183 

158  "       waves  or  flashes 176 

224  Auroras,  annual  inequality  of. 198 

223  "           "       periodicity  of. 189 

37  "       dark  segment  of. 178 

11  "       distribution  described 186 

9  '                "           explained „  199 

27  '       diurnal  inequality  of. 198 

'  "       periodicity  of. 188 

210  '       duration  of. 177 

11  '       geographical  extent  of. 177 

43  "  in    both    hemispheres    de- 

76  scribed 188 

10  "  in    both    hemispheres    ex- 

78  plained 200 

235  "  in  Southern  hemisphere —  188 

236  "       recurring  fits  of 177 

236  "       secular  inequality  of. 198 

191  "             "      periodicity  of. 189 

177  "       solar  spots,  etc. ,  on 282 

193  "       within  the  tropics 200 

185 

184  Ball  lightning 167 

186  Banks,  temperature  of. 51 

173  Barometer,  accidental  variations  of...     21 

173  affected  by  the  moon 20 

180  "          air  excluded,  how 12 


302 


INDEX. 


Barometer,  Aneroid 

"          capillarity,  correction  of.. 
"          column  measured,  how. ... 

"          construction  of. 

"         corresponding   to  boiling 

water 

"         dependent  upon  height. ... 
"         diurnal  variation    ex- 
plained  

"          extreme  fluctuations  of... 

' '          fall  in  hurricanes 

' '         falls  under  a  cloud 

"          Hardy's  self-registering.. 

"          height  at  all  hours 

"  "      in  different  months 

' '          heights  measured  by 

"  "  "        —tables 

"         high  near  lat.  32° 

"          Hough's  printing 

' '         hourly  variations  of 

influenced  by  the  wind 

influence  of  latitude  on — 

low  near  lat.  64° 

"      "     the  equator 

mean  height  of. 

monthly  means 

observations,  how  repre- 
sented   

"          reduced  to  freezing  point. 

* '          self-registering 

"         temperature,  correction  of 

"          two  daily  maxima 

Barometric  depression,  amount  of.... 

Biela's  comet,  division  of 

Breezes,  land  and  sea 

Cirro-cumulus  clouds 

Cirrus  clouds 

Climates,  marine  or  continental 

Climatology 

Cloud  formed  by  condensed  vapor 

Cloudiness,  average  amount  of. 

Clouds  classified 

defined 

formation  of. 

height  of. 

how  electrified 

"  sustained 

indicate  currents  in  the  air — 

mode  of  observing 

negatively  electrified 

peculiar  arrangement  of. 

shadows  after  sunset 

shadows  of. 

vertical  thickness  of. 

Cold  which  causes  hail 

Comet  of  1866 

Contact  arches  described 

' '  variable 

Coronte,  cause  of. 

"  colors  of. 


I'aire 

15 
14 
13 
12 

265 
22 

62 
21 

281 
138 
16 
263 
262 
22 
260 
84 
17 
19 
21 
18 
147 
147 
18 
19 

138 
258 
15 
14 
63 
139 
235 
86 

102 
101 
39 
23 
137 
103 
101 
101 
104 
103 
165 
105 
106 
102 
166 
106 
107 
106 
104 
133 
234 
222 
223 
215 
214 

Coronas  described  

214 
215 
101 
84 
207 
150 
147 
150 
151 
149 
148 
148 
149 
148 

10 
194 
120 
120 
119 
238 
2  tO 
239 
240 
93 
91 
92 
91 
59 
57 
93 
205 

44 
46 

46 

45 
44 
45 
200 
161 
160 
195 
166 
164 
161 
164 
162 
165 
161 
165 
163 
164 
163 

163 

163 
162 
160 
62 

'  '      produced  artificially  

Cumulus   rlonds,,..,. 

Current  ascending  near  lat.  64°  

Cyanometer  

Cyclone,  premonitions  of.  

Cyclones  defined  

diameter  of.  

duration  of.  

gyratory  movement  of.  
originate  where  

paths  of.  

rate  of  motion  of.  

season  of  

Dalton's  theory  of  the  atmosphere  
Dark  segment,  cause  of.  

Desert  of  Africa  

"       Gobi  

Deserts  enumerated  

Detonating  meteors,  examples  of..... 
"         multiple  nuclei.. 
'  '         number  

"              "         periodicity  of.  ... 
Dew  amount  of  determined  

"     circumstances  favorable  to  
"    during  the  dav   

'  '     origin  of.  

'  '     point  deduced  from  wet  bulb.  .  .  . 
"       "     defined   ,         

"     where  unknown  

Displacements  bv  refraction  

Earth,  fluctuations  of  temperature.... 
"      frozen   stratum  

"      increase  of  temperature  

"      maximum  and  minimum  tem- 
perature                   

"      temperature  at  different  depths 
great  depths.... 
Electric  circulation  system  of  

Electricity  at  considerable  elevations 
atmospheric,  how  observed 
circulating  about  the  earth 
discharged  to  the  earth... 
diurnal  change  of  

"       variation  of.  

due  to  evaporation  

in  cloudy  weather         .... 

in  dry  houses  

monthly  change  described 
"      explained 
result  of  combustion  
"       condensed  vapor 
'  '       friction  

"       unequal  temper- 

"       vegetation  

variation  with  altitude  
Electrometers  

Elevation,  influence  upon  humidity... 

INDEX. 


303 


Evaporation,  amount  measured 

at  all  temperatures 

"  rate  variable 

Evening  sky,  redness  of. 

Factors  for  wet-bulb  thermometer.... 
Fog  bow  described 

"  "  explained 

Fog,  diameter  of  particles  of. 

"  how  sustained  in  air 

Fogs  constituted  like  clouds 

"     in  spring  and  winter 

"     of  polar  regions 

"     over  rivers 

"     where  most  prevalent 

"  where  unknown 

Force  of  wind,  how  measured 

"  "  represented  by  a  scale 
French  feet  converted  into  English... 
Frost  in  valleys 

Gaseous  atmosphere,  annual  variation 


Humidity,  extremes  of., 


p»g« 
55 

56         "          of  air,  relative 
55         "          of  the  air  denoted 

206  Hygrometer  defined. 
Bache's 

273  Daniell's, 

98  "          Saussure  s 
214 

99  Ice,  anchor  described 
99  Ice-houses,  natural 


of. 


Gaseous  atmosphere,  diurnal  varia- 
tion of. 

Gases,  law  of  mixture 

"  proportions  of,  in  atmosphere 

Glaciers  described 

"  geographical  distribution  of 

Glow  surrounding  shadow 

Gulf  Stream,  influence  of. 

Hail  attended  by  two  currents 

"    circumstances  of. 

"    formation  of. 

"    formed  at  what  height 

"    geographical  distribution  of. 

"    how  sustained  in  the  air 

"    parallel  bands  of. 

"    preceded  by  a  noise 

"    quantity  of 

"  rods 

Hailstones,  form  of. 

how  long  sustained 

size  of. 

"  structure  of. 

Hail-storms,  track  of. 

Halo,  how  a  circle  is  formed 

"     of  22°  radius 

"     of  46°  radius 

"     of  90°  radius 

"  theory  of. 

Halos  described 

' '  produced  artificially 

Heat  lightning 

Heat,  radiation  of. 

Hemispheres,  northern  and  southern 
Hoar-frost,  crystalline  structure  of.... 

how  formed 

Hot-springs,  observations  of 

Hourly  variations  of  barometer 


Pupa 
60 

274 
60 
56 
57 
58 
56 


53 

49 

52 

100 

[nterval  between  flash  and  icport 169 

283 

.  34 


Ice,  polar. 

Indian  summer  described. 


[ron  meteors,  catalogue  of., 


96  Isothermal  lines 
97 

69  Kilometres  converted  into  miles 253 

70 

254  Lakes  and  rivers,  temperature  of. 

94  Latent  heat  liberated. 
Light,  absorption  of. 
Lightning  caused  by  volcanoes. 
63         "         color  of." 

different  forms  of. 
duration  of. 
origin  of. 
tubes . 


62 

10 

11 
126 

127|Magnetic  disturbances,  cause  of. 

215         "  progress  of... 

146         "        needle,  disturbance  of 

Mercury,  depression  in  glass  tubes 

133  Meteoric  orbits 

129i       "        streams,  origin  of. 

134  Meteorology  defined 

132  Meteors,  detonating,  defined 


of  November. 

of  November,  1866. 

"  1867. 


131 
134 
135 
133|Metres  converted  into  feet. 

1 30  Millimetres  converted  into  inches 

135iMirage  at  sea 

130  "      described 

134       "      experimentally  illustrated 

129       "      lateral 

131  "      upon  a  desert 

132  Mist,  appearance  of. 

217!Monsoons,  cause  of. 

216          "  "     of  uniformity  of..... 

218  "          described 

219  Monthly  means  of  barometer 

216|Months,  hottest  and  coldest 

216|Moon,  effect  upon  barometer 

2 18  Morning  twilight,  colors  of. 


KIT 


Motion,  relative,  from  earth's  rotation 


89  Mountains  enveloped  in  cloud. 

37 

94  North  pole,  temperature  of. 

93jNovember  meteors,  conclusions  from 

47  "         elements  of. 

19l         "  "        period  of. 


53 
137 
206 
172 
168 
167 
168 
166 
172 

197 

190 
190 
263 
230 
237 
9 

238 

230 

231 

232 

252 

25} 

204 

201 

204 

205 

202 

98 

85 

147 

85 

19 

38 

20 

1109 

82 

104 

36 
235 
233 
233 


304 


INDEX 


November  meteors,  procession  of  node 

of. • 

November  stream,  dimensions  of. 

Pacific  Ocean,  two  sides  of. 

Parhelia  of  22° 

"        of46° 

"        of  120° 

Parhelic  circle 

Pillars  of  sand 

Plants  protected  from  frost 

Pluviameter  described 

Pointed  objects  tipped  with  light 

Polar  winds,  direction  of. 

Predictions  founded  on  constant  cli- 
mate  

Predictions  founded  on  the  laws  of 

storms 

Predictions  founded  on  observations. . 
"          of  the  weather  possible — 

Prognostics  from  clouds,  etc 

"  "     twilight 

Radiating  power  of  different  substan- 
ces  

Radiation  from  different  substances.. 

' '         with  partial  exposure 

Rain  affected  by  elevation 

latitude 

"          mountains 

"  "          proximity   to    the 

ocean 

"         winds 

amount  measured 

annual  fall  of. 

diameter  of  drops  of. 

distribution  over  the  earth 

for  each  month 

from  clouds  not  in  zenith 

' '     translucent  clouds 

gauge,  exposure  of. 

great  annual  fall  of. 

greatest  fall  of. 

how  caused 

Huttou's  theory  of. 

in  the  different  months 

maximum  fall  of. 

origin  of. 

or  snow  in  a  storm 

size  of  drops  of 

small  annual  fall  of 

without  clouds 

Rainbow,  conditions  of  visibility 

"        described 

' '        result  of  interferences 

Rainbows,  supernumerary 

"         theory  of. 

Rain-gauge,  proper  height  of. 

Rainy  days,  number  of. 

Rainy  season  and  dry  season 

Redfield  and  Espy's  theories 


Pag* 


Page 

sea-breeze  in  temperate  zones 87 

232  Sea,  currents  of  the 51 

234    "    surface,  temperature  of  the 60 

"    temperature  at  great  depths  of  the  50 

37    "    temperature  of  the 49 

221  Sheet  lightning 167 

221  Shooting-stars,  altitude  of. 226 

222  described 225 

220                            direction  of  motion  of  227 

153                            length  of  path 227 

94  light  of 228 

108  magnitude  of. 227 

173  '  number  at  different 

75  hours 225 

'  number  for  the  globe  229 

157  '  "in     different 

months 226 

158  '             periods  of 237 

158  '              soundof. 228 

157                            telescopic 229 

159  '              visible  train 228 

210  Showers  of  toads,  fishes,  etc 156 

"       remarkable  examples  of. 119 

Sky,  blue  color  of. 207 

281    "    reflected  light  of. 207 

90  Sleet  defined 129 

90     "    origin  of 135 

114  Snow,  annual  amount  of 123 

113      "     avalanches  of. 128 

115  Snow-balls,  natural 125 

Snow-flakes,  form  of 124 

116  "           how  formed 122 

116  "           size  of. 125 

108  Snow  from  cloudless  sky 122 

117  Snow-line,  height  of. 273 

108  Snow,  perpetual  limit  of. 42 

111      "     phosphorescent 125 

278      "     red,  in  polar  regions 126 

121      "     where  unknown 123 

121  Spaces,  interplanetary,  temperature  of  43 

1 09  Springs,  ordinary  temperature  of 47 

280  Storm,  American  example  of. 142 

118  "      defined 136 

110  "      European  example  of 141 

111  "      lull  at  the  centre 144 

117  "      wind  on  the  borders  of. 144 

1 1 5  Storm's  progress  and  direction 1 43 

108  Storms,  cause  of 136 

140       "       parabolic  course  of. 151 

213       "       rate  of  progress  of. 140 

280       "       rise  and  decline  of. 140 

121        "       their  course  modified 145 

211  Stratus  clouds 102 

210  Surface  wind  in  middle  latitudes 83 

213       "          "    in  polar  regions 83 

211  "       winds  defined 76 

212  "          "      in  equatorial  regions..  82 
110 

113  Telegraph  wires  affected  by  storms...  173 

118  "      auroral  influence  de- 
146     scribed 191 


INDEX. 


805 


Telegraph-wires,  auroral  influence  ex- 
plained  

Temperature,  change  from  latitude... 
"       with  elevation.. 

daily,  defined 

decrease  explained 

different  latitudes 

for  each  month 

great  absolute  range  of. 
' '      monthly  range  of. 

highest  observed 

hourly  variations 

increase  of,  with  height 
interplanetary,  its 

amount 

invariable  stratum  of... 
"  law    of  decrease    with 

height 

lowest  observed 

"            maximum     and    mini- 
mum  

mean,  above  80° 

"       below  18° 

"       from  three  hours 
"     two  hours. 

"       of  a  place 

monthly    changes     ex- 
plained  

monthly,  determined 

near    the    centre    of  a 

storm .. „ . 

observations    at    -single 

hour 

range  of. 

small  absolute  range  of 
"      monthly  range  of 

succeeding  a  Kiorm 

variation  at  Greenwich. 

"         New  Haven 

variations  non-periodic. 

"        cause  of 

Thermometer  described 

exposure  of. 

graduated,  how  — , — 
hourly  observations  of. 

maximum 

Phillips's  maximum... 
photographic  register.. 

requisites  of 

self-registering  

wet-bulb 

Thermometers,  Centesimal  and  Fah- 
renheit  

Reaumur    and   Fah- 
renheit  

Thunder,  cause  of,. 

"         clouds,  height  of. 

"         duration  of..-,,. 

"         rolling  of........ 

Thunder-storms,  distribution  of. 


Pag«i  fmg, 

j  Thunder,  succession  of  phenomena  ...  170 
197  Tornadoes,  appearance  of  explosion..  153 


effects  of  .....................  152 

examples  of  ................  152 

in  the  tropics  ................  152 

Trade  winds  described  ...................     74 


34  j  Twilight  curve 


268 

272 

271 

39 

29 

90 


14.- 


209 

"        duration  of. 208 

Upper  current  in  equatorial  regions. .  82 

"         middle  latitudes....  78 

"        polar  regions 78 


Vapor,  annual  variation  of. 61 

43      "       diurnal  variation  of. 61 

45      "       elastic  force  of 276 

how  formed, 54 

41      "       how  sustained  in  air 54 

39  "       of  atmosphere  condensed 95 

weight  of,  determined 60 

29  Vapors  distinguished  from  gases 10 

270  Velocity  of  wind  deduced  from  press- 

270  ure 69 

31  Vertical  columns  through  the  sun 224 

30  Vesicular  theory  of  fog 98 

33  Volcanic  ashes,  prevalence  of 100 

Volcanoes,  information  derived  from.  47 

"         show  direction  of  wind 76 

Waterspouts 154 

Waves,  atmospheric 139 

Weight  of  air,  dry  and  saturated 264 

30  Wells,  temperature  of. 48 

40  Whirlwinds  caused  by  fires 154 

272  Widmannstaten  figures 245 

271  Wind,  average  velocity  of. 70 

145      "      direction,  how  determined 65 

267      "             "          "     indicated 64 

266      "     effect  upon  the  barometer 21 

33      "     mean  direction  of 71 

27      "      on  different  sides  of  a  storm...  140 

23      "     pressure  and  velocity  of 277 

27      ""      temperature  of. 87 

23  Wind's  direction  observed 73 


28 
25 
26 
26 
24 
25 
59 

255 

256 
168 
171 
109 
170 
172 


progress,  how  represented 72 

Winds    affected  by   rotation   of  the 

earth 81 

caused    by    unequal    specific 

gravity 80 

causes  of. 79 

cold,  from  mountains 88 

hot,  from  deserts 88 

how  propagated 81 

influenced  by  the  seasons......  86 

in  the  middle  latitudes 74 

on  summits  of  mountains 77 

propagated  by  aspiration 144 

three  systems  of. 74 

transport  fine  dust 77 


u 


PLATE  I. 


VAN  RENSSELAER  HARBOR. LATJB'37' 
V 


GODTHAAB,  GREEN'D.   LAT.64-°JO' 


NORWAY  HOUSE, H.B.TLR.  LAT,  S5° 0' 

X 


ST. JOHNS, KEWFOVJD.  LAT.47  3-5' 


NLW  YORK  CITY.  LAT.40°42' 
TSf 


SAVANNAH.GA.  LAT.32°6' 
IT 


"W- 


MATANZAS, 


GEORGETOWN,  B.fi'A.^r.eVa'/v. 


PLATE  II. 


Cirrus 

2*1  Cuimilos 


STORM    OF    FEB.   5     I67O. 


PLATE  III. 


721 


QC 

861 

L8?t 


Loomis  -  A  treatise  on  Meteorology. 

UNIVERSITY  OF  CALIFORNIA  LIBRARY 

Los  Angeles 
This  book  is  DUE  on  the  last  date  stamped  below. 


FED  6 

JAN  1  8  RECO 

APR  1  1  1962 
APR  2  8  ROT 


FEB  1  3  1968 
FEB20 


HKT 


RECO 


JUN  X  X 


JUN  4 


APR  H  1972 
JUN  XX  197^ 

MAR  -  71978 


Eug  MT4 


Form  L9-50m-9,'60(B3610s4)444 


DNIVEJBSITY  of  CALIFORNIA 

AT 

LOS  ANGELES 
LIRJU 


A    000169859    6 


